Picked up a Diamondback Cobra 20" MTB for my 7-year-old son last weekend. Fits him well, but he can't downshift. It came with a SunRace grip shifter. I called the local bike shop and they recommended replacing it with a SRAM MRX Comp shifter. Brought it home, installed it, adjusted it....he still can't shift. Just doesn't have the grip strength required.

Is there an easier shifter to mate with a Shimano SIS 6-speed rear derailleur? I use SRAM thumb shifters on my bike, but I'm wondering if his little thumbs can reach the buttons, and strong enough to pop them down to the hardest gears?

Or should I just tell him to live in 3-gear until he develops some grip strength?