If you are looking for an economical solution, I'd just go with Shimano triple front shifter. If he forgets and shifts into the missing gear, the limit screws on the derailleur (make sure they are setup properly) should keep the chain on track. I think I paid $25 at the LBS for the triggers I bought to replace my old grip shifters, and they are working great for my use. Personally, since it's a junior bike, swapping out all those components seems like a lot for something he will outgrow at some point.

Also, make sure you pay attention if you go with a SRAM unit; some of them pull different amount of cable than Shimano, at least the last time I was looking at that stuff.