He wouldn’t say save the pet. He would probably say that morally in this situation where similar interests are at stake- both the non-human animal and the human are about to die- then the choice to save either is equally valid, unless you can identify something about saving the child that doesn’t happen when the pet is saved. If we want to say that those who would save the pet are morally wrong, then we need to identify what that something is.
For example, a rational argument might be, when the pet dies then only the “owner(s)” and potentially other family members in the household (including other pets) may suffer as a result. If the child dies, the parents, grandparents, aunts, uncles, friends, teachers etc may all suffer. However that’s not actually a logical distinction, because in theory you could have the single parent of a newborn with no family where only one person was going to suffer. So what else can you look to.
Lifespan is another way to try and rationalise that only the child is a valid choice. The child will almost certainly live longer than the pet. But that rationale only works if you would save the pet if their lifespan was more e.g. if child had a terminal illness.
Instinct is pretty obvious and most people’s instinct would be to favour their own species or empathise with them more. It’s not rational or particularly logical but in a choice between two equal harms it’s a a sufficiently valid basis for a personal choice, but not to say another choice is wrong.
Level of suffering might be a way to rationalise it, but it’s hard to prove- in mammals, is there any pain worse than a parent losing a child? Do people really grieve as long for non-human animals as they do humans?
As I say I’d save the child. I made the decision nearly automatically and then tried to justify it with lifespan or suffering related arguments but really, it’s just because I empathise with humans more, which is not rational, but when the stakes are exactly the same I don’t need a logical basis for the choice. I think level of suffering probably also factored into my decision, as I find it hard to imagine the loss of a dog could ever be as painful as the loss of a child.