It all depends on how cranky your kid gets. If your kid will be happy playing in the sand on a beach and hanging out for a few hours making friends with fish in the coral reef lagoon, then yes. If they say, "Can we go?" after 5 minutes, then no.
DC is like visiting Key West, it's a place you go to relax, not chase a 4 year old who doesn't understand how much money you're spending to be there. With 12 people, I'm sure someone will watch the kid, like the 8 year old. The only problem is they both might run off, and then you'll have to constantly watch where they are. You can lay down and relax, but you'll always have to keep an eye on them.