I had something similar the other weekend, I was backing off my girlfriends driveway, it backs on to a main road so naturally you have to go slowly anyway, plus adding into the pathway to the road is sloped downwards, so any kids walking past are even harder to see if they're short.
So I start slowly backing off and I hear a scream, like you I slammed on the brakes, I looked in my rear view mirror, still couldn't see anything, looked out and saw 2 kids pop their heads out and a full 5 seconds later whoever was walking with them appeared. So basically he was letting them run off down the street and just monitored them at his own pace, he tried to tell me to "be more careful" and not to "come flying out" next time, I apologised and then told him it works both ways and said that kids that young and small shouldn't be left on their own anyway, plus i can't "fly out" as its a main road anyway.
Irresponsible idiot.