Because a child signals are potentially easier to understand, due to the parent and the child being from the same species.

Kind of funny, we did make a similar choice (although only in terms of which one first. We wanted children) and the feeling of being incompetent dog owners was one of the reasons pro "no dog"

Best to bring a dog into a family with a child, rather than bringing a child into a family with a dog.

The child needs to be higher in the pecking order, as seen by the dog.