And what about dogs that bite a child (weather defending themselves or not), that usually get euthanized because of this?
I think it's a crap excuse to get rid of a pet just because you're having a child. The pet was there first, why act like it's expendable? I feel the only reason that would be excusable was if the animal was already overly aggressive.
The second issue usually lies with parenting. I feel bad for animals that were never once aggressive beforehand, getting put to death because a kid was taunting or abusing it in some way. If the parents let the child demonstrate behavior like this with their pet, and the child gets bitten, that's their fault.