We were in DC as a family, with 3 daughter’s last summer. A few thoughts–
First, why not let them pick what they want to see? Two of my three favored the natural history museum, one loved air and space. The american history one was closed at the time. Or since they are all close, you can do combinations-- we did only the 1st floor of natural history (roughly all things animal, skipping all things rock), then went to air and space. Check content online, and see what you want… its all free.
A few other oddments-- we only let 2 of our 3 see the holocaust museum, one is too young. They didn’t love it. I went to the washington monument with the 3rd… and we both loved it. For either, look up online to get tickets in advance. Its a small mail-order fee that saves you the hassle of standing in line for tix. All my family enjoyed the Lincoln memorial end of the mall. Make sure to find time to do that. It’d be my opinion you’d be better served seeing the lincoln memorial and taking a drive to arlington nation cemetery for the changing of the guard… but that’s my opinion alone.
And for anyone reading considering a DC trip… remember, you have to write your congressman or senator for white house tour tix and capitol tours now. Allow 4 mos, minimum. White house tours, to us, were good… capitol tours are at the whim of an intern… and ours wasnt worth the time, sadly.