You should have broken a window, got the kids out and taken them with you to the police station! smile
I had a similar experience, minor the car when I was younger. 2 kids (only about 1 and 3) running around the carpark at work (which was on the corner of 2 very busy roads right next to a 5 road roundabout!) I ended up taking them inside and trying to work while looking after them. The poor things were starving, the sausage sizzle fundraising people outside gave them each a sausage and I ended up giving them my lunch. And found colouring pencils and paper to try keep them occupied. I phoned the police who said they would send someone. I looked after the kids for about 4 or 5 hours while waiting for the police! Then a women comes out of the bar/TAB from next door (this is like 6 hours after the kids were left alone) and snaps at them that theyre going, and then says "I supose you want some food" and the lady from the sausage sizzle said she had given them each a sausage so with no thanks or anything the lady grabs the kids and walks off. I was pregnant so not about to chase them as who knows what she would have done to me but I was pretty pissed the police had never shown up! (even more so considering their station is the other end of one of the roads my work was on and it would have taken them like 5 minutes to walk to me!!!) Felt so sorry for the kids.
Fingers crossed you get a better result!