What to do after an accident with a child?
Where I live there is a legal requirement to report any injury accident to the police, and from past experience they would call a bleeding little finger an injury. However, I personally would not bother for such a minor injury, they will fill the report and forget all about it. 

As far as other legal issues (Again, speaking from my own) - you stopped, ascertained injury and damage and rendered assistance - you fulfilled you legal obligation. 

It appears that you did way more than was really needed, and had/have real concern for the other person -  there is little else anyone can do, especially if offers of assistance are turned down and advice is shunned.  

On the issue of lights and helmets - there is not much you can do about it and you went beyond what is morally required (Sounds like mum probably thinks way beyond). 

A couple of weeks ago a friend of mind had a teenager (No helmet) T-Boned her car at an intersection - he came off a foot path at high speed and could not stop. He and bike flew over car bonnet leaving a small dent, he cracked his head on landing. Although dazed, he refused offers of help and a ride home etc, and rode off before an ambulance was called (Here there is no cost for medical treatment for accidents), despite several bystanders "suggesting" that he needed to be checked out. What else could she do?  These things happen - especially with kids and teenagers, as long as no one is seriously hurt, it's a good day