This is entirely my opinion, but I would strongly encourage you to turn that video over to the police. Not to get someone in trouble, but to get both the mother and the child help. The reason that children lose their lives everyday to poor parents is because good people recognize signs and let them go because they assume it's none of their business.
Great concept to mind one's own business, but not here. That child needs an advocate. Consider that the parent tried to dump her kid on a total stranger. Not in a million years would I separate myself from my child, and I have four of them. That little girl will end up as collateral for debt to some dealer. The mother has already demonstrated she will willingly give up her kid to save her own neck.
I have a friend who investigates child cases for CPS. What he encounters on an daily basis will twist your mind. You really do not want what he sees in your head. Most of the time they know it's happening, but lack the hard evidence to act. You have that evidence.
Get the mother some help. For all you know there is already a social worker desperately looking for evidence to warrant the appropriate action.
Turn in the video and go about your life knowing you may have saved someone elses.