There are probably many ways to try and teach this, and social skills are difficult to teach in any approach, not just ABA. With ABA, you want to control as many variables as possible in order to ensure that the change in behaviour is a direct consequence of your teaching, in order to keep the teaching as efficient as possible. However, you cannot control the variable that is the 'other person'.
One of the ways I would approach this I think is to avoid telling him what not to do, and start to teach him the skills that are more appropriate, using the fact that he is motivated to interact.
You'd start slowly and probably role play the scripts at home and practise, practise, practise, offering praise (or whatever motivates him to keep on task) when he gets things right, and especially when he comes up with his own ideas.
Then, you'd go out to a playground and attempt it in real life. If he can stick to the 'plan' then.
The plan has to be just 'one' thing probably, like saying hello, for the first time. And once he has got that, then try the next thing.
Instead of walking around hoping he doesn't do the wrong thing and avoiding small children, you have a plan, he is clear about it, then you go out ACTIVELY looking for opportunities to practice the skill. You praise, and reinforce all the things he does correctly and stop when he is doing well and succeeding, before the situation gets strained and he starts to get negative feedback from the child.
Then you work on what could happen next, and go out and try that.
You could also watch children from afar and get him to comment on their interactions.