I once ran into a little girl of about 3 or 4 as she ran out from between two parked cars. I swerved and slammed on, avoiding running her down but clipping her head with the wing mirror - luckily I was only driving slowly (20 zone) so she was mostly just shaken up.
As [TW]Fox mentioned, the first thing that entered my mind was to floor it and get out of there as quickly as possible - and it was quite a powerful thought. I considered legging it and hesitated for about 10-15 seconds before I got out to check she was okay and had her mum and dad rant a bit until a witness pointed out they were at fault for letting a kiddie run loose. Gave them my details and never heard any more about it.
I didn't run off but I can fully appreciate why some people do, especially if they think they've killed someone.
Just noticed that an O.D. may to blame rather than a hit'n'run - still, regardless of cause, my condolences to the OP and their family.