Collective nouns for cats: a clutter, a clowder or a pounce. Last one is totally the best, no?
For kittens: a litter, a kindle or an intrigue. Again, I like the last one the best. Even if it makes sound a little bit as though the kittens should be gracing the covers of gossip magazines 
Am sure we could come up with some better collective nouns though.
A snuggle of kittens, maybe?