My 7 year old girl is big, and very strong for her age she rides a Trek mt220. I has 7 on the rear and 3 on the front for 21 total. SHe rides on the middle ring on the front, and is free to use the 7 on the back as she chooses. If we come to a very steep hill, I will tell her to shift to the little ring, and she has never used the big ring. Except for some really big hill, she'd be fine without the triple on the front.
It didn't take her long to figure out the gearing, sometimes she still tends to spin out lower gears as she is climbing mild hills, but she does ok.