My 20th is this year too. We were going to splurge on a vacation, but now I think we'll be headed back to the East Coast for my niece's HS graduation instead. (With our 2 kids, who will be 10 and 4).
I live in CA, so I guess I would suggest Disneyland - it's sort of compact. There are many places to stay - we've only been once, but we stayed at a place that was condos/ suites. Our suite was a 2BR with kitchen, had a king bed in one room, with room for our porta-crib (our younger son was two), and two full size beds in the other room. The couch also was a sofa bed, so the place slept 8. (My MIL and I slept in the room with 2 doubles, and hubby and 2 boys in the king room). This place also had a shuttle to Disneyland right out front.
I can also recommend Legoland, which is a little cheaper than Disney. Again, you can get hotel suites or condos nearby, many with shuttles. (We stayed at a hotel this last time.) Legoland is smaller and cheaper.