I doubt the puppy attacked. More likely is that the baby is in the swing, feet dangling, and the puppy wanted to play. That's how puppies are.

If they put the put to sleep for that, it's just insane.

I'm sure the kids are upset that their child is dead, but they're just kids having kids. They have no clue sometimes how quickly an incident can escalate. The mom probably thought she could relax and get a nap and just let the baby go in the swing or something like that.

It's just a tragic situation for all concerned.