i would call the people who take care of bad parenting
I would cry
It would depend on how old the kids were and how far from home they were. Plus it would depend on if the mother came back for the kids and so just doing it to make them behave.
I would walk up to them and ask them if they needed help of any kind, and if they said no I would press it (because they obviously did), and I would offer to either walk them home or somewhere safer.
If I were grown up, I would say "HEY! THOSE KIDS ARE SCARED TO DEATH! THAT'S CHILD ENDANGERMENT, IF I WERE YOU I'D SAY SORRY AND TAKE THEM HOME!" Or call the police