Yeah, it is a really thin case - lots of rules for this "child benefits for grandparents" too. Legal adoption is required which is hard to do when the parents are still alive and the grandparents are the ones adopting, have to have been dependents for at least a year before grandparents draw social security, kids really are dependent, and on and on. Happens to be that for our family, all of the boxes appear to be checked once the adoption is finalized, which it likely already would have been if not for the pandemic closing courts in my parent's state for non-essential business. Of course we'd all rather this social security thing wasn't happening for the kids - that likely means a much happier situation for everyone overall.
Anyway - on the numbers, Dad's benefit is 75.6% of mom's benefit (a little less if she waits a couple of years but still >> 50%), so it makes the most sense for them to each collect their own benefits. Parents are married.
What I'm getting at is 0.5 + 0.5 * 0.756 = 0.878. If you put both kids on mom's record, which if you have to pick just one or the other obviously makes the most sense, you run into that family limit so the incremental benefit from kids is just 0.75. Assuming that is allowed, and I think that it is but was hoping someone here might know a lot more than me, that's $400 / month or so better than both kids on mom's record. $1K / month better than both kids on Dad's record.
This is a largely academic exercise - my mom is also getting a fairly generous pension, I added it all up under various scenarios yesterday and they'll certainly be fine regardless of the choices made - the "worst case" as far as the total income is concerned - they choose the 50% survivor benefit on the pension, mom dies first, the kids are old enough there is no social security for them (actually this part of it is a very good thing - means mom lived for another 13 years or longer from today), mom takes social security immediately and so gets the lowest benefit there - all of that happens and my dad would still have over $6K / month between the pension and social security in that scenario. Both the pension and social security are inflation adjusted. In the short term, the monthly total is into 5 figures regardless of what they choose once mom starts drawing the pension and social security. And they're both working still - mom doesn't retire until end of August and I think plans to work some part time, and Dad is working part time as well. They are set. They don't feel that way, but if they can't make this work financially it isn't because there isn't enough money coming in.