Doseage--Is 20 XR too much for a child?

Does 20 XR sound like a high dose for an 8-year-old...I guess I am wondering if this is very unusual? My son started on 10 mg IR about 3 weeks ago, last week we went up to 15 mg (10 xr with an additional 5 IR in the morning). Then tomorrow we are going up to 20 mg XR. It does seem to me that it will be the right dose, but I have not read much about other children taking that much. He immediately did wonderfully well on Adderall when we started 3 weeks ago...within in an hour his teacher emailed me that he had been the star math pupil that day and was completely focused. He even said that he likes life better focused! But right from the start it wore off quickly, like in 3 or 4 hours. The XR lasts longer but it is clearly not quite enough. We hoped the 20 XR will be it. Anyone else out their have a child on this much? For years I refused to start the medication...I was just afraid for him, afraid of the side effects. But the last 3 weeks have proven me wrong. I still hope this is not a forever solution, but it is helping now. I know that supposedly weight has nothing to do with it, but he weighs 92 pounds. Thanks. J.