Unless the child is trained in a specific feat or some specific magic is used, there is no "protective creature that is automatically struck in place of the target creature" according to the rules as written. But nevertheless…
The coverage rules include a +4 to automatic AC for another creature that provides coverage, giving a +4 to AC. Many GMs play this when a fault of 4 or less hits the top, instead of the target creature, although again this is not part of the rules as written. More importantly, the significant difference in size between a very young human and a small rat could give you a margin to declare that the rat has improved coverage (+8 to AC, and you would be entitled to say that it is missing by 8). or less hit the child) or even total coverage and / or total concealment, making attacks against the rat impossible.
As a unique effect, having the first shot hit the child when he is submerged on the road is fine, although damage such as precision damage (in the hope of preventing the party from accidentally killing the child in a single attack) should not be applied. If you can imply that this will happen ahead of time in some way, either generally or as a result of a skill test (Sense Motive or Perception, perhaps), players should not feel that they were deceived simply because there is no specific rule that cover. that. That's point In the GM after all, things that are not programmed as a computer RPG can happen.
If they continue to attack despite the +4 or +8 penalty and they kill the innocent child at that moment, it is their fault, and although I am not a fan of the "unique" effects that affect the alignment-based classes, this could be easily an exception, in my opinion.