I mean, they're small creatures that people love and want to take care of. They're not the same as children, of course, and I don't think most people think they really are. I certainly don't, and no matter how much I love my dogs if I had a kid I'm sure that I'd love them a lot more than any pet I've ever had. I think for most people it's a cutesy nickname for their animals more than anything.

That said though I know calling pets children can be really annoying to a lot of people, so it seems like something you might want to use more sparingly than some people do.