We have two cats and have never had a problem with them jumping in the cot when the kids were little.

We didn't do anything in particular, when they were really really small, we had the bedroom door shut when they were in there sleeping, but the cats never went in there anyway. For a few months before DS was born we made sure that the door was shut at all times, so the cats sorta got the hint that it was a room that was off limits to them.

The only time our cats have been a problem was the other day when MIss M was taking a nap (she's 14 months old) and she was woken up when the male cat went into our room and jumped in her cot. However, we had the side down because it is next to our bed, so it was easy access. If the cot had the side on it, I very much doubt the cat would have gone in there.