NGE,
Everyone has given good advice. Let me share how I made my choice for my son's first bike.
He was going on his 9th birthday, about 4'4" and 95 pounds. He rides bicycles, skateboards and can snowboard. So he has a sense of balence and velocity.
I wanted to get him a bike with a clutch. He will need to learn anyway so why not start him off that way. I wanted something that would last a few years. It needed to be user friendly in the begining, but would keep up with him as his skill level grew. The bike was gonna be a tad bit big for him now, nothing that would be unsafe mind you but it needed some growing room. I chose a 4 stroke for ease of maintenence and it's smooth power. I was on a budget, aren't we all, so used was the way to go.
I took him to the local dealer a few times and had him sit on different makes/models. Just keep in mind, on average, a newer bike will sit a little higher than it's used/older counterpart due to suspention settling. My first bike was a 1973 XR-75 and I have fond memories of that bike, so I settled on hondas XR line. Just a personal preference. We tryed the XR-70, he was a tad too big for it. He fit the XR-80 perfectly, no growing room there. We tried the XR-100, he was on his tippy-toes. I thought this is the bike.
I found a 1996 XR-100 for $700, it was a little cosmetically worn, but in sound mechanical shape. His first ride was a blast. He had to kickstart it while resting on the kick stand. After a few tries he got it. He didn't even stall it on his first take off. He wound up riding for about 2 hours, doing figure 8's in first gear.
As for maintenence, let them get involved as much as you can. It's takes longer "supervising" but they will learn responsibility and respect for their bikes.
I look back now, as he's hitting the jumps in the back yard, and think about that first ride. That bike was a perect choice. He's had the bike for 3 years now and it's kept up with him. Plus he has lots of fun on it.
Hope this helps some, Steve.