This is all so wrong in so many ways. How the heck did he manage before he met you? Who paid all his bills then? Where is all the rest of his money going? You should not be expected to pay for HIS children! You are perfectly right that you are only responsible for your own children. They are your top priority. 

If this millstone is dragging you down into poverty then I suggest you get rid of it ASAP. You can still date him but you don't have to live with him.

Have you taken note of the airline safety briefing: secure your own oxygen mask before helping others?
Well this is pretty much the same: take care of your own kids first, before helping others.

Look I am pretty much pro-skid and very pro-fairness, but you are being utterly unfair to your own kids in  this situation.
The skids have two parents, your kids have you.
If you were able to comfortably provide for your kids prior to your SO and his kids, but are struggling now "on two incomes" then you see the problem.

Your SO is having a life of Riley at your kid's expense: they are being deprived by your choice to play Mother Theresa.
If this were my situation, I would tell SO, that I can not subsidise him and his kids, not one single penny.

Either he contributes fairly to the requirements of him and his kids in your home or he needs to live elsewhere.
If he doesn't like it, know he loves your purse more than you.

Please don't do this to your kids.
Not for a man.

Admittedly I'm American, but how is 320 a month breaking the bank for him? Is he underemployed? If so, he needs a better job or a second job.  How was he paying it before he met you? Where is your money going now, to pay for him and his kids in your home? 
 

If he's okay with you supporting him and his kids to the detriment of your own, he's NOT a keeper. 

Step 1, unentangle all finances. He can pay you the rent for the month and if he can't, then he needs to move out

Step 2, a deep dive into finances if you're planning on marrying this guy. 

Step 3, once steps 1-2 are complete, you do not mesh finances again, even once married. You have 2 small children who are wearing shoes with holes in them. YOU are responsible for putting a stop to the money in/money out situation for them and they're the ones suffering right now because of your attachment to this man. Truthfully, both you and he are making some questionable financial choices at this point to the detriment of your kids.

Step 4, this guy honestly doesn't sound worth it. I am sure you can find someone better suited to be a better life partner.

I would give him 30 day notice to move out tbh, you have to prioritize your kids needs. You tried living together but this situation is unsustainable. You can live apart and date until your boyfriend is in a better financial position.

Then I would check that I was getting all the benefits that I was entitled to. 

Then try to find other resources that can help you. Perhaps ask at the children’s school if they know of any local charities that can help, or try the citizens advice brureau or your gp’s office or your local mp - especially in this election time they want your loyalty (you don’t have to give it, but you are still entitled to assistance). If you can find a food bank, they might be able to help with finding other local resources as well as food for your family. 

What you are doing is not acceptable.  You went from being a single woman able to support her children and meet their needs to one who is not meeting their basic needs, and for what?  A man?  That's not fair to your kids.

You need to separate finances.  NOTHING should go to your boyfriend and his kids until your kids' needs have been met in full (and I'd say you also have an emergency fund).  What did he do before you came along to subsidize him?  A self-respecting man who was a good partner would be perfectly amenable to splitting finances so that you were not supporting him.  If he's not, then he's just using you.

In any event, put YOUR kids first.  That's where your responsibilities lie.  No money for him and his until the kids have everything they need.  

It's time to pull your funding from your bf.  Seperate finances.  I GET wanting to combine, but it's negatively affecting YOUR kids.  Also time to cut outings down.  Your kids deserve a lot better, and he's taking from them.. He needs to fund his own kids.  If he doens' thave money for support and gas, then he needs to get a second job or pick up more hours.  Plain and simple.

Sorry you're facing this

There should be no way in the world your kids should go without..NONE. This man is a liability and not adding to your life in any way, shape or form. You cannot accomodate him, his lack of finances and kids while sacrificing your own. You need to separate finances and he needs to go. You are your childrens ONLY source of income and this man is bleeding you dry. Personally I'd give him a time frame to get his stuff and leave. Your kids are paying for this dearly and he has no care in the world that YOUR children are going without.

No way in the world that I would allow that to go down....

Hope you make the necessary changes to get your life back in order, before this man came in and has you struggling.

You are not being unreasonable.  Is someone making you feel that way?  Your kids don't deserve this and only you have the power to change the circumstance.  I imagine you felt blending finances would help you financial situation not hurt it.  My Ex boyfriend moved in with 1/2 custody of his 3 kids.  Immediately began making excuses about money.  Claimed he thought he didn't need to pay half of rent because he bought groceries.  I said there are 4 of you and 1 of me, no dice.  Kicking him out of my place was difficult because he fought so hard and gas lighted me but it was the best decision for my life.

No way. You should be paying your share of the household bills not half your BF's total financial obligations. Food, rent, services ... that's it. Why are you contributing towards his kids but he won't do the same for you? 

since he moved in with me and my children my finances have changed for the worse

Time to give him the boot.

Take your next paycheck and buy your kids what they need.  Then put the heat on in the house so that they are not going to bed cold.  And with your next paycheck buy them Christmas gifts and the late birthday present.  Do not put a penny in a joint account or hand him any cash.  He can take care of himself.  If he can't pay his bills, tough sh*t.  If he is not adding to your life then he has to go. 

No man in your bed is worth your child walking around with holes in their shoes.  Do you really need a stranger on the internet to tell you this?  What would you think of another woman doing this to her kids?

I'm so sorry that you and your children were drawn into this. (Sorry for his kids too, as well, but they should not be the OP's problem except in a global "my brother's keeper" sense.)

Your situation embodies my view of the core, gut-level, nearly instinctive issue that makes steplife untenable for most. You and your offspring are, in the most literal and concrete way possible, competing with him and his for resources. There is no bond of blood to soften things among you. (And with that many in a household, even the closest of relatives would have problems.)

Stop it yesterday. Take your kids and just get out. Get off the bus, Gus. Make a new plan, Stan. You may have hoped that many hands (or four, anyway) would make light work. Now you find that the extras complicate matters. You and your children will be far better off without this lot burdening you.

Good luck.

You know what to do so do it.  Quit complaining and act.

Purge this failed parent and partner and his baggage from your life and the lives of your children.

Equity life partnership has to benefit both partners.

I have the same issue since moving in with my partner. But we both have one boy each and a boy together. I had plenty of money for me and my eldest before we moved in together and had our son but now!! I have had to cut back on work to stay home with our 3yr old, who goes to day care on the days that I work, in Aus daycare costs a fortune!!! My eldest goes to a private school that I pay for solely by myself. I still have bills to pay and now have to buy groceries for extra people and literally had to start putting my sons school food in a seperate cupboard so his son wouldn't eat it all on the weekends! My outgoings are more and my incoming less.

My partner used to put money in an account for groceries and things for our son but stopped yet still expects me to cook him dinner every night. 
I understand he is the main earner but he can't grasp the fact that that is because I CANT WORK BECAUSE SOMEONE HAS TO LOOK AFTER OUR SON!! I don't get paid to do house work arghhhhh!!