Hee hee. Itty bitty kitty bites. Go Dante; EAT HIS SKULL!!
If Kaylee really wanted to keep the kitten a secret, then she'd not have put a bell on it. The custom of putting bells on cats is to keep them from catching birds; cruel if you ask me.
If I didn't know better, I'd say that they're not keeping it from Mal. I'd say that he knows all about it and is pretending not to to annoy Jayne. ...or to keep the thing around as an auxiliary food supply.