I don't trust cheapie bikes. Kids are hard on things and I just feel safer with them on well made, professionally assembled bikes.

My boys have 16" Specialized Hardrocks. They just came through their 3rd summer on them and they are in like new condition. For their 7th birthdays they are getting new 20" bikes - they are asking based on color, so i am currently investigating the brands I trust (trek, specialized, gary fisher, giant, marin) to see who makes a good 20" with handbrakes and gears in the colors they have requested. You need to watch what you are buying, some 20" bikes are what are called Transition bikes. They have foot brakes and one handbrake. They are good for large, younger kids (ie, a 4 year old who needs a 20") who do not yet have the fine motor skills to use handbrakes full time and to use gears.

My daughter has a 12" Giant. She was adopted at 21 months old and did not have good gross motor skills when she first came home. She was never able to master peddling her tricycle. One day we were in a sporting good store, hopped on a Giant 12" and started riding around the store. It has been a great bike. She has had it for about 13 months now and it is in great condition.

You can check out reviews of kids bikes at moutain bike review, www.mtbr.com