Federal law mandates that your spouse is the beneficiay on your 401k, unless the spouse signs a very specficially worded post-nup (has to be post nup, not prenup, as only spouse, not future spouse can sign). 

This only applies to 401K, not IRA, so if you terminate service (change jobs or retire) and move money to an IRA, that can be left to a child, subject to state laws (which generally require that some portion, generally 1/3 to 1/2 of specified assets be left to a spouse).  The state law issues can be dealt with via a prenup. 

DH and I signed pre nups and post nups that our respective kids get each of our 401Ks. 

I think counting on a new spouse to take care of your biokids is unrealistic.

I don't have kids, but DH's beneficiary is 100% me.  My SS is 19 and BM still gets CS, which would undoubtedly still have to be paid by me if DH died before SS turned 21.  And after 21, he's an adult and on his own. 

I personally think spouses should be beneficiaries if someone is married.  401K is meant to be a retirement investment that should go to a surviving spouse, and if something is left up on that spouse's death, perhaps something goes to the kids on both sides.

Our will does say that SS gets 75% of anything left over when both of us die. If he ends up estranged, that might change. 

This can be a sticky issue.

Generally, I think that married couples should leave the bulk of their wealth and estate to their spouse.  However, I do understand that this is a risk because if your spouse remarries and predeceases their new partner.. your children could end up with nothing from you in the end.  Or, your spouse could intentionally leave your kids out of your will.

On the flip side, we don't know what the future brings and your spouse could NEED that money to survive and would that be your wish to help your spouse actually live through a medical crisis etc.. before you would leave funds to your child (that may have a whole career in front of them to save etc...)  Your wealth might be tied up in a house too.. and what happens in THAT situation.

For these reasons.. and others... it is usually a good idea to speak to someone who can guide you through these waters (if it's significant enough to make that worth while)  financial planner.. estate atty etc.

One mechanism to help this might be use of trusts.. which makes the money.. or proceeds from the investments available to your spouse while they are alive.. but when they pass.. the principal balance goes to your children.

In my case.. my DH is 100% the beneficiary as I have no children and we have a will in place that will give me 100% of everything.. the intent is that if he predeceases me.. I would pass on wealth to his children.  We are over 50.. so at this point, I don't "realllly" see me remarrying if something were to happen.. and even if I did, I would still set aside a reasonable amount to pass to his girls..

But, we can't always count on people doing the "right thing".

But what do you do?  I guess it depends on how much you trust your DH to take your daughter's interests into account.  How much will he "need" this money too.  If he is very well off on his own.. maybe it stays with your DD?  Otherwise, you could always make some split to beneficiaries..

Spouse is the automatic beneficiary to 401(k) unless they sign a waiver stating otherwise and you file it with the 401(k) record keeper.  Even if you have your children listed, once you remarried it will automatically go to your spouse unless they fill out the waiver.  This is a federal requirement.

If you are young at time of second marriage, and most asets are minimal, it is reasonable to expect that assets earned during marriage go to spouse, with some to kids.  If you are older, going into marriage with significant assets, not quite the same thing.

I'm more of a fan of setting up trusts if you want to pass on wealth or funds to kids after you die. I think retirement accounts are too unpredictable, especially if you are married. You could outlive your 401K, or you may be destitute at some point and never get to retire, having had to spend all your savings to survive.

If It's important to you that the kids have *something* when you're gone, then I would look at other options, such as a separate, smaller life insurance policy that they are the beneficiaries on, setting up a trust or savings account for each of them that you and DH put a little money into each month, opening a long-term CD in their names, etc. A post-nup is also a good plan to ensure that the kids get an equitable share upon your individual or mutual deaths.

All children should realize that they aren't guaranteed money upon their parent's death. I wouldn't feel good about taking part of my mom or dad's 401K if one of their SOs was still alive and could use it. They mutually worked for it, not me. I don't want to see my SF or Dad's GF struggle in their elder years just so I could buy a new car or pay off my house.

There are tax implications (estate and income) to this, too, although unless you have an estate exceeding the rather large threshold for estate tax that one won't come into play.  You might want to look into that or talk to a financial advisor.

My DH gets nothing if I die, everything goes to DD.

And before I had my child my beneficiaries were my best friend and cousin. I would prefer to give everything to the charity instead of leaving a penny for the DH’s kid and I am sure he would benefit from my money. I started from zero and worked hard enough to have USD 350 K savings and two apartments on my name. 

DH chose to quit studies, work, mate and reproduce with an uneducated woman, so be it. 

How are you going to live after you stop working at 60 what ever ?  Social security $1800 a month is not going to cut it unless you plan on eating dog food .  

Others have made some of these points above, but I'll share my two cents: 

1) 401Ks / IRAs are meant to cover retirement costs. One challenge with covering your needs in retirement is that you don't know how long it will last and can't predict what your expenses might be. In my view, leaving some of this money directly to a child and by-passing a spouse is risky, because what if your spouse needs funding for health care or other support services after you die? A stepchild is not likely to cover extra costs and prevent that spouse from falling into poverty. I think spouses should plan to provide for one another when they cannot work. Children can presumably work and support themselves after their parents have died (assuming that children are adults when their parents die).

2) If a parent wants to leave something to their child, they should save for this above and beyond what they are putting aside to cover living costs after they retire. You should only look to pass on wealth to your children if you can cover your own needs and the needs of your spouse first. The whole - put on your own oxygen mask before helping others - idea. There is no requirement that a parent financially support a child after that child has reached the age where they are capable of financially supporting themselves. It's a societal convention to expect an inheritance. It's weird to me when people feel entitled to money that their parents earned. I tell my parents repeatedly that my "inheritance" will be knowing that they were taken care of and could do whatever they wanted before they passed. I'm not looking for them to give me a large chunk of money. They earned that money, I didn't. 

3) My DH has these great dreams of both paying for SSs to go to college and giving them a chunk of money when they turn 30. He has not done much saving for either. He and BM saved money for college and then BM cashed out the accounts because she "needed" the money. I've told DH that I have not been saving for children, so if he wants to do either of those things, he needs to find a way to save for those things he wants to do - without shorting his retirement savings or his contribution to our household expenses. I've been saving for my retirement and yes, I will not allow DH to go without during retirement, but he also has to be planning and contributing. It may be sad when SSs don't get anything, but I don't see it as my job to make sure that either of their parents are financially planning for them. 

I don’t understand how people on Medicare with a supplement end up with boatloads of medical bills. I am on Medicare and my total medical expenses are well under $500 a year. I see 5 different specialists and take 7 pills a day.  My supplement premium is $0.  I am not low income so this is not subsidized.