I'd ask the hostess, because I know for my shower, I *wanted* kids there. I don't get to see some of those kids very often and I liked having the kiddos present. It wasn't super formal (a backyard) and so it wasn't inappropriate to have kids present. I specifically told my hostess that I was okay with it and that she could spread the word to those of my guests with kids that they were welcome to bring them. (A few were even specifically invited on the invitation--if they were old enough to read it.)
Most of my friends then brought their little ones, except two who didn't want to chase them around because having them there was more work for them and they wanted the afternoon off!
But other showers are more formal and kids would be out of place, such as at a restaurant or tea house. So use some judgment on that too.