Shopping Advice (scroll down past the advice to the products if you like)
Overview - These are for seven to ten year olds who want to ride from A to B with both wheels on the ground. If riding without parents, riding only short distances, or if hooning around is the main purpose for the bike, a BMX bike could be a better choice.
Gears - They'll have from 3 to 21 gears, supplied either by combining, six or seven sprockets at the back with a single or triple chainring up front, or by having internal hub gears (often combined with a back-pedal brake). More gears means smaller jumps between gears and a 7(or 21) speed drivetrain will usually be of higher quality than a 6 (or 18) speed, giving smoother and more reliable shifting..
Gear Levers - Small hands cope well with twist grip style gear levers. Also, thumbshifters are simple and reliable, but not quite a s easy to use with small hands.
Brakes - Most will come with V-brakes. These are powerful, giving plenty of stopping power. Bikes that have internal hub gears will often have a back-pedal (coaster) brake, which is great for simplicity and intuitive braking.
Brake Levers - Make sure the brake lever reach is OK, so the rider can comfortably reach and operate them. Many brake levers have adjustable reach. Adjusting the lever reach will mean readjusting the brakes. It's difficult to adjust brakes to a very short reach lever; slight brake rub on the wheels can be difficult to avoid.
Weight - All else being equal, a lighter bike will be faster, easier and more enjoyable to ride than a heavier bike.
Tyres - If the bike will mostly be in the dirt, a knobbly wide tyre will be good. If it'll usually be on sealed roads, consider a narrower, smoother tread tyre. If the bike doesn't have suitable tyres fitted, don't worry, they can easily be changed.
Bike Size - The rider not only needs to be able to stand comfortably over the frame with feet flat on the ground, they also need to be able to steer comfortably while seated. Check this before buying.
Alloy Rims - Alloy rims don't rust, and brakes work reasonably well in the wet on alloy rims. Steel rims rust, and give hopeless braking performance when wet.