It sounds like you took on too much of a "parental" role.

Im 25, just about to marry, together 3 years. My SD is 5, BM and my fiance were never married and had split when SD was newborn, so they had 2 years prior my arrival to sort their crap out.

Obviously I can only go off what you've written here but to me, this whole asking them what they want for breakfast and making it, doing their washing, making sure they have sweets/snacks/treats, essentially doing little things to spoil them during their time in your house, has become too much. And the reason for that could be many things. It might be as simple as the skids are starting to see the differences in what YOU do for them and what their BM does for them (or in this case, doesnt do). Imagine being in BMs household in the morning with the kids asking for something for breakfast "well melektra would make it for us" what is BM going to say? She will say something negative.

And whilst its normal to want to do nice things for our step kids so that "we are friends" its rarely turns out well.

I cant help but ask how you started taking on so much of the day to day role. Where is dad at breakfast? Why isnt he the one ensuring there are snacks/treats for when his kids come to visit if thats what he wants?

But also, why are you making them breakfast at all?
My SD is 5 and she gets her own breakfast, she knows she can choose between cereal, fruit or breakfast yoghurts. Occasionally we will have a warm breakfast but thats on a rare basis and offer only.

But whatever, regardless of why this is the position you are in now, you cannot change what has already happened but you can determine how to deal with it moving forward.
Id be having a sit down with your partner and telling him that since the kids feel you are some "big, mean, scary" person then you are taking a massive step back and make sure he is backing you in doing so.
When the kids next come over you both need to sit down with them too and dad needs to lay the law. Have him tell them that hes really disappointed that they think so negatively of you, explain to them that if you were mean you wouldnt have done all those things for them. You need to actively deal with this negative opinion, they need to be explaining why. You need to give them examples, at 10 and 7 they are old enough to be able to do this.
Ask them, if they didnt like someone would they do X Y Z? How would they feel if dad told them that he thought they were mean and scary? Would that hurt their feelings?

I am a big believer in being honest with kids in an AGE appropriate manner.

And I love the plate analogy; take a plate, drop it on the floor so it breaks, say sorry to it, does it fix the plate? Yes you can glue it together but it will not be the same.
Explain about feelings like that, when presented with consequences for their words/actions they will possibly turn on waterworks and pull the sorry card.
You can take that apology depending on how truthful you believe it to be but you still take your step back. The days of reloading from you are over. From now on its down to dad to provide for them, if you feel the motivation to do something with or for them then you invite them to join you, but you cant/dont force them to join in.

First quit letting this toxic BM and her rancid crotch nuggets steal your happiness. Quit catering to them. They are not infants. Quit asking them what they want and start telling them what they will do.

Set the rules in your home and hold them accountable for following those rules including standards of behavior and respect. If they choose to violate the rules and standards... bring the consequences down on them and make them rue the day they violated the rules. Ill behaved kids get only misery. Well behaved kids get a shot at an enjoyable experience in your home. Teach that lesson consistently and be happy while you do it. }:)

The best revenge to take against people like this BM and the toxic spawn she dropped with your DH is to live well and be happy. They can be reasonable and participate or they can live with the misery of the consequences for their decisions to not participate.

Your job is to be happy, and brind the pain when necessary.

Good luck.

Ok, you are ME when I first joined this site.
I did ALL of what you are doing.
The only difference is that DP did stick up for me big time a couple times to set the SDs straight.

We are doing well now, with little hitches along the way, and are able to talk rationally about SDs, expectations, and behavior. And he actually TOOK MY ADVICE the other day! Wow! We'll see if he follows through...

You've got to just stop doing it. No money, no time, no "extras."
You figure out what YOUR basic needs are in your household: clean house? quiet house? A good, grown-up food dinner once a week? OK to do laundry but not put it away? Dishes? Table/restaurant manners?
I made a beginning list of 3-4 items that I would NOT disengage from (clean shared living area, no SDs in bedroom without permission, no using furniture as gymnasium equipment, all laundry IN laundry baskets or it doesn't get washed) and stuck with it, gradually adding little things here and there after discussion with DP about rules.

It will never be easy - sometimes it's fun, we have good times together, but the heavy parenting is on the parents, not you. Get used to saying, "Ask your dad."

Get the book Stepmonster so you can understand the dynamics. It's not going to solve the problems, but it will make you aware of what's happening.

Today is the day they arrive, hold that head high and never let em see you sweat!

If they think they got your number then guess what, they did!

I always think after an "incident" like yours that you should disengage a few weeks later so it can't be blamed on this issue. Make sense? This also lets the kids think "oh, everything is going to be the same", until BAM, they are hit in the face when they want you to get them pizza and you say "I'm sorry, I'm busy, you'll need to go ask your dad to help you". BEAUTIFUL!!!! OR you walk in eating a pizza SLICE, lol. This way they know you ate without them.