That’s . . . disturbing.
On to your actual request.
There could be a lieutenant themed around pin-the-tail-on-the-donkey, who is blindfolded and flails about while randomly stabbing heroes.
You could have some sort of stage magician enemy, who has illusions and teleportation and perhaps an army of rabbits.
Another option could be some minions made out of animate balloons or streamers, or both.
There could be some innocents trapped inside of a giant birthday cake that the heroes will have to rescue from being sliced up by an automated knife-robot when a timer runs out.
Or, alternatively, burrow something from one of the Adam West Batman episodes and have a giant cake made out of quicksand. (It makes just as much sense in context.)
Environment ideas could be a ball pit or a bouncy house, both of which would likely reduce the damage from attacks.
As for your main villain, here are a couple of things you could do with him:
- He siphons the fun and enjoyment from partygoers into himself. (Archetype: Inhibitor)
- He steals gifts from others to use for himself against the heroes. (Archetype: Inventor)
Also, a resource I would like to recommend using is this excellent Environment created by @FrivYeti.