You can’t exactly share a 529 as far as I know, so if your kids are spaced less than four years apart it could be a pain figuring out how to manage those expenses. Of course you could just pay a year or two for one kid before passing it down, etc.
We have separate 529s for each kid. I really doubt they’ll leave any cash in either, but if they do there are plenty of ways to transfer the money while avoiding taxes. I think I even crunched the numbers and taking the 10% penalty and cashing out isn’t even necessarily a money loser if you had the money invested long enough.