And they had no way to know whether the child was alive or not. That is the point. Too many people are thinking of this from just the aftermath instead of what the decision is in that moment, not knowing whether the child is alive or dead or anything else we do know now. Hindsight is 20/20.
It was too dangerous to enter at that moment. They were trying to get the fire more contained. Plus, they had no idea where to look for the child, yet they would have known exactly where the father was had it made it through the door. Plus, there is no telling what he might have done had he started on fire himself (became a "screaming alpha"). A person who is on fire can start to panic and spread the fire other places, including outside the house.