It really did help to have the feedback not be coming from me. That way, I could be the coach without having to also be the judge. AoPS is nice because the instructors and quite willing to tell you what's wrong with your solution when it's wrong, but they also are willing to give positive feedback unleavened with "yes but"s if it really is good. I think they have a good feel for the "right" amount of encouragement for kids.