Since this is a Recreational & Family forum, it seems to me to make sense to have a discussion about what size bikes fit what size kids.
So to get it started, here are pics of my three boys on their steeds. Sorry they're so big (and blurry)! The camera was focused on the fence across the street while they rode by and I tried to click at the right time.
Anyways, the the point is simply to get an idea of fit. Feel free to tell me whether I have them on the right size bikes, upload pics of your own kids, or just read to help decide what size bike to get your own kid!
Here's my #1 on his Mongoose MTB with 26" slicks, seat all the way down. He's 60.5" tall.
Here's #2 on his new (to him) Haro V-Bone 24". The seat could maybe go down another inch or so. But I'm hoping this bike will last him 3 years until I can put him on a 26" like his big brother. Right now he's 52.75"
And here's #3 on his Trek 60 (6 speeds in the back, one small chainring in the front). Obviously, the seat is pretty high, but I think I can get another year out of it. He's 51" tall.
I think the bikes fit the kids just fine. My biggest complaints are:
Twist/grip-shifters are hard for little hands, and that's what's on bikes #2 and #3. Are triggers that much more expensive?
The chainring on the Trek 60 is SO small (but seems par for the course with this level of kids bike) they can't get up any decent speed, and have no chance to keep up with their brothers.
Knobbly mountain tires are also holding them back. Why can't kids bikes be outfitted by default with more like a commuter tire?
For a conventional diamond frame bike, there should be about 1 inch between the top tube on the bike frame and your son when he has his feet planted firmly on the ground and he is standing over the frame and off the seat. The simple rule for seat height is that the child's leg should be almost (but not quite) straight when the cranks are in line with the seat tube. That gives the maximum transfer of power to the pedals and doesn't strain the leg muscles. The pictures seem to show that you have the seat height correct for all three boys. I often see kids with the seat so low that they don't come close to using the most powerful part of the stroke and they insist that is the way they like it. Take them on a long ride and they are often the ones that fall behind.