Wow - that would have ended badly for everyone involved! If ever there was a good illustration for the value of safety systems, I think you've captured it!
The kids face after a stern warning says it all. Glad you didn't hit the little bugger. Kids are like cats. They need nine lives, because of all the stupid things they do before reaching adulthood, requires lots of good luck and fortune!
This is why I really appreciate an excellently implemented ADAS system on dash cams combined with modern ADAS system built into cars.
Having clear evidence to prove your innocent in case of accident in extremely valuable, but having an ADAS warning system combined with emergency braking with pedestrian detection and a good set of tires to prevent accidents is even more valuable.
Your astute attention and quick reactions saved that young child from a young child's mistake. Great driving. I don't think they make the Highway Code in a board book for 3 year olds edition... and even if they did I don't think that kid would read it until this happened anyway.