My 8-y/o son was constantly frustrated with the shifter on his 7-speed 20" K2 Arapahoe. It was the standard cheap grip shifter that you see on low-end bikes and it was something that was pretty stiff for adult hands and really too hard for a kid to use.
So I decided to get a Rapidfire shifter for him. I tried the local bike shop, but they weren't much help. So I ordered it from Amazon... bundled with some Christmas shopping to get the Super Saver Shipping:
Shimano SL-M310 shifter.
I think there's an 8-speed version available, too.
Anyway, it came in, so one evening I put it on his bike, but I didn't tell him. So just this last weekend, the weather was nice and he was going to go for a ride. He loved it. Later, we went out to the local park and did a little mountain biking, quite successfully, too...
When I was looking for a shifter, I wasn't sure which ones would be good for a kid. This one is. Get it!