Family dogs also tend to take on a protective role over children, since they're pack animals and correctly identify the tiny humans as fragile/vulnerable. Dogs will offer the kids as much love and attention as they're willing to take without ever getting bored, while adults often have their own limits and need to deal with other Important Business that children don't necessarily understand/appreciate yet.
Yep. I could see choosing to save a dog over a person on similar lines. A person should be able to take care of themself and would (generally as an adult) not be the responsibility of another person. A domestic dog may not be equipped to take care of itself and as an owner you feel a moral responsibility for their wellbeing.
It's not surprising that the child would lean towards protecting the animals that they see as under their care, or needing more care than adults.