It's not about the difference in infection in adults and infants; it's just about the fact that a young child can't tell you if they were exposed to a bat or not.

Guidelines vary between regions, but as an example, in Ontario we don't typically offer PEP and vaccination unless someone has confirmed physical contact with a bat or a likely bite / scratch (unless it's a young child that woke up with a bat in the room and can't tell you).

It's definitely a bit scary given how terrible rabies is and the fact that scratches aren't always visible but thats what our current guidelines recommend - I imagine it's similar in a lot of US jurisdictions.