An interesting question over who to blame. You could consider it a freak accident. The mother was temporarily distracted, the baby crawled off, the father had no idea that she was directly behind his vehicle.
Should you check the path of your vehicle before reversing? Probably. Do people inspect under every wheel? Probably not. He could have walked round and seen that the path was clear, hopped up into the cab (I assume it's a stupidly impractical giant truck), played with his phone for a couple of minutes while the AC warmed up, then pulled out.
Probably everyone's to blame, or maybe no one. Sometimes these things just happen.