The best way to get someone to realize that they're doing something wrong and change their behaviour is to state it to them in a friendly, non-confrontational way. Assuming they're just being oblivious, and not downright mean. So I guess you could have said "Sorry, you probably don't realize it, but the way you've parked my son is forced to ride out into the road, but the road really is too dangerous for him at his age."

But I wouldn't have I would have yelled and said something rude. Even though all that generates is a bit more animosity.