wow, I guess I shouldn't have thought I was the only one. My son is 8 and is doing the same thing, although he is only doing 1 to 3 SNGs a week. He has always watched me play and watches poker on TV with me and a couple of months ago I let him play under my account. He doesn't have as high of an ROI as your son, but he is in the black.

My wife doesn't have a problem with it, but I think as long as your son is still active in other activities that can't be much of an argument and you will find that it gets them more interested in math.