I’d have to side with Warsuit on this one. 21 is not a kids party. Those are adults and to be treated accordingly. These are the club-goers and unless you are Paris Hilton (foam) or Steve Aoki (cake) there is no reason to go for the all kinds of games.
Your music selection should be doing the talking. If the parents asked about the fun and games, they are -imho- way behind the curve and/or have forgotten how desperately they didn’t want to be treated like children anymore at 21.
As Warsuit said, we might be way off the mark here and not understanding some local or regional customs at those kind of b’day parties, but my advice would be to focus on preparing your music, making sure you’ve got all the avenues covered and ready to move the crowd in the right direction.
And don’t expect everyone to be up and jumping at 7pm. Not sure if there will be alchohol served at the party. They ARE 21 after all. If there is, that should make your job a tad easier as the evening progresses. Start slow with that finger-snapping, toe-tapping, head nodding music that puts everyone in a feel good mode and build it up from there. If you can get a real party vibe going from 10-12, I’d say you’ve done a good job. That gives you 3 hours to build up the floor.
Another thing to find out is if the principal party-goers are planning to hit the clubs after the party. It makes a difference if you are the closing act for the night or if you are effectively a warm-up for the clubs that follow the party.
Hope any of this is help. That is what we are here for.
Greetinx.