I think a 7 year old is certainly capable of operating the camera, especially on Auto or P. I don't think the average 7yo will quickly master the concepts of depth of field/freezing/blurring motion, exposure triangle, etc. You have guide mode, so that will be nice when he is ready. There is a big variation among kids that age on how they will handle a moderately delicate piece of equipment. Some will treasure and protect it, some will toss it around like any other toy. Since he already has a P&S, you know he is responsible. I would still keep the P&S around though and not talk it down. If he enjoys taking pictures, sooner or later he will want to take the d3100 other places. Then you are in the position of deciding whether or not to let the camera/lens go on a class trip or to a friend's birthday party, etc. Kids are kids and no matter how responsible yours is, things go downhill in a group. So keep the P&S around for those times.