My son is 12 also and he has a 2004 Blaster. He has the Toomey pipe and we are both very happy with it. I have heard the LRD pipe is very good too. Check out
http://www.atvflorida.com/forum/index.php/topic,9392.0.html for more pipe info.
Regarding your idling problem: First check to make sure that the starter plunger (i.e. choke) is all the way in (assuming its already warmed up). Then, tweak the idle adjustment screw on top of the carb (remove the seat for access) as required for a good idle. Turn it clockwise to increase idle speed. If that fails, inspect the rubber joint between the carburetor and reed valve for cracks, loose clamp, etc. and repair as required. If it still won't idle, replace the spark plug for good luck and then its time to tear into the carb. I can walk you through that if need be but hopefully the idle screw will do the trick. Good luck and let me know how it goes.