Hey, I'll put my two cents in since my boys owned 7 different bikes so far,
When you decide to let your Kido start riding a bike, you need to evaluate what kind of riding your going to do and what type of personality you Kido has. If you want your Kido to ride trails or your land has a lot of steep hills then you might what to look into a four stroke, like the Honda XR's or Yahama TTR's. I would'nt worry about the gears because like the other post said you can just keep it in a gear that is most comfortable for your Kido, then as he's confence level grows he';; start switching them him self. If your growing up a motocross racer then you';; want to get a bike that has lots of suspension travel, Like the KTM JR 50's, Husky Boy JR 50, Polini X1 50, or LEM's LX 50. These bikes only have one speed, and a soft suspension, On an MX bike brakes are important two, I would'nt start your boy with hydraulic brakes their too easy to lockup and skid, start with a bike that has cable brakes, and something to think about is brake location, some bikes have the brakes on the handle bars and some have the foot brake, It's much easyer if your Kido starts with hand brakes, there's just too much going on to have him use his foot to work the back brakes, If your Kido rides a bicycle with hand brakes then he'll best use hand brakes when he rides his dirt bike.
Now a harder decision you'll have to make is the power level you should be looking for, and this has a lot to do with your kiddos personality, I brake it down into three types, the mellow, the tame and the wild, for instance, You don't want a bike that will spin the tire or has a power band that hit's too low in the rpm's if your Kido is kinda mellow, it'll scare him and he would want to ride it,. If your kiddo is alittle tame and not a scared cat and can ride he's bicycle like a dirt bike (jumping ramps and curbs, turning real fast, and doesn't mind if he carsh now and again) then he'll enjoy being able to spin the tire and get the bike rev'ed up a bit, If you Kido is a wild one then you'll want to be extreamly careful, as these type will take on anything you throw at them, maybe he can pull off bunny hopes on he's bicycle or tries to pull wheelies, and will ride anything including a mountain bike that is way too large. Start him off with a race bike and tone it down (a lot), and as he's skill develops tune up the power. It'll happen very quick, trust me.
Then there's different bike body styles, some are skinny at the tank some are fat, some have soft seats some don't, some have disc brakes others don't, some have low profile tires some have fat tires.
In my sons case, he's a tame rider, he liked to spin a tire now and again (in a straight line of course) or trying to do little jumps. He likes fatter bikes with fatter tires and brakes he could lock up. His first bike was a 2002 husky 50, which had an air cooled morini engine same as some KTM's. Both brakes were on the handle bars and used them good. His second bike was a KTM jr pro 50, this bike had a very quick power band and when it hit I'd spin the tire, and was skinny at the tank, with a hard seat, he really did'nt care for it, third bike was a polini X1 and was the bike for him, He still liked he's Husky but he quickly realized that the polini had a better feel and alittle more power, so I bought another X1 for he's race bike. When it was time to ride a SR bike he went though a KTM SR pro 50, then a Beta 50, then a king cobra, then a polini X3, then a LEM R3, Every bike except the X3 had a wicked power band and clutches that were washer type not spring, so he stayed with the polini. We've also owned and XR 50 and a ttr 90
these were lame to him since he started on a two stroke, switching gears was no problem, and it took little instruction to get him to use the gears, but he would do the stuff he did on he's two strokers (like jump, or try and do a standing start like at the begining of a race, he just rode them.
If you do decide and get your Kido a race bike there are things you can do to make it easyer to ride, First, make sure it has a sping type clutch, there easy to adjust, and engage much softer than a washer type. You may also look for a bike with a soft seat, It took me awhile before my boy strated to stand up and ride like your suppose to, so a softer seat will not hurt the butt like a harder on and your Kido will ride longer, If you get a race bike that is too fast then try getting a smaller carberater, and changing out the gearing (sprokets) two our three teeth on the spokets make a huge difference on a single speed bike.
Don't get a bike with hydraulic brakes. If your bike comes with skinny tire in the front try and get him a fatter on,, the skinnier the tire the easy it'll be to turn and your Kido will do better with a fat one that take alittle effort to turn. another good idea is to drop the front spings as far as they'll go into the triple tree, (make them touch the Handel bars) this will aid in he's turning. And the skinner the bike is'nt always better, I think it has to do with how far apart their legs are, the fatter the bike at the tank the better I think, it's a balance thing. (hold your knee's together you have less balance bring them apart you'll have more balance.)
It's a tough decision you'll have to make, I just kept buying bikes and let him decide which one he liked..