Also, how do you define a child, when the driver's decision can only be based on appearance? Plenty of 'boy racers' (and girls) around here in their mid-teens who are technically children, but under helmet and glasses could be anything from 12 to 25.
Wouldn't it be better (and arguably more inforceable) to make it obligatory to pass any cyclist/pedestrian/horse allowing at least a specified gap (as, I believe, is the case in France) of, say, 1.5m? And better still for the HC to say something like you 'should only overtake when it is safe to use the opposite carriageway (on standard A, B & lesser roads) when the speed limit is more than 30'?
Overtaking distance is as much an issue as speed, and personally I'd hope drivers would be trained to slow down to pass any 'obstacle' (worst round here are drivers coming towards you on single-lane roads who barely slow down and expect you to leap into the verge/gravel for them).