First of all it is illegal:
It doesn't matter to me that she stopped and gave out her phone number. She has an obligation to make sure he is o.k. and assist him if he's not. It seems clear that he was NOT o.k.
But beyond the criminal aspect, it's morally reprehensible to hit a kid on a bike with a car and just leave him there! When you go through something like this your adrenaline is pumping and you might be in shock. You can't know for sure if you're injured.
I hope that Brandon recovers from this quickly and the woman who hit him pays for his medical bills and a new bike. Whether she deserves criminal prosecution is up to law enforcement, but it seems to me that she did not fully live up to her obligations under the law.
Get well soon Brandon,