What to do after an accident with a child? I was biking home last night and had a minor bike accident with a child of around 12 or 13. It was around 6pm on a mixed use pedestrian+bike path and completely dark, with little lighting on the path.

I was going uphill maybe going 8mph, he was on the downhill. I was well lit (headlight + white flashing light on the front of my bike, with full reflectors including reflective vest), he was unlit, no reflectors (except half a yellow reflector on one pedal). I saw him at the crest of the hill, he was on his side of the path, I was on mine. Then not more than 10 or 20 feet in front of me, he suddenly swerved into me. I was against a fence and had no where to go, so I yelled at him and just clamped on the brakes as hard as I could before he hit me.

I think the left side of his bike clipped the left side of mine, but both of us ended up on the ground, he seemed ok, and got up on his own after laying there for a moment -- he did have a bleeding finger, but he was able to bend it without pain. He wasn't wearing a helmet.  I asked him where his mom was and he said she was biking nearby, so I asked him where I could find her. He called her on his friend's cell phone (who also had no helmet or lights or reflectors) and told her to come. 

I asked if he was ok or if he hit his head, he said he was fine and didn't hit his head. Mom showed up a few minutes later (no lights, reflectors or helmets) and said "What did he do?" so I told her what happened and pointed out that he needs to have reflectors and a helmet, and she said "but it was light out when we started riding". I also told her to check him for head injury just in case he hit his head on the ground, and she kind of rolled her eyes at me. My bike was ok, I was ok, he seemed ok, and mom just wanted to leave so I just left it at that and continued home without incident (I did discover the next day that my head light was scraped up and the light mount was cracked, but I had a spare mount so that was easy to fix), but after leaving I'm wondering what else I should have done?

We didn't even exchange last names, I got one picture of him and his bike with my cell phone camera but that's it.

Should I have called it in to the police? California law requires riders under 18 to wear a helmet and to have lights at night. Should I have said more to the mom about helmet and reflector use? Should I have gotten his full name in case I discovered significant damage or injury later? 

Or since neither of us was injured was just biking away the right thing to do? 

This is my first real accident in over 15 years of riding, I always figured that when I did have an accident it would be with a car, I never thought that it would be with a snot-nosed (literally) kid on a bike path.
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