- Proper fit. The child should be able to (barely) reach the ground with their toes. If they can stand nearly flat-footed while on the seat, the seat is too low. If the seat cannot be adjusted higher and they are still on training wheels, that is a good time to use the painless two-wheeler training method. They must be able to stand over the frame.
- Bearings in the headset (fork bearings) rather than bushings. Bushings stick, and make it far harder to balance. If you don't plan to transition to a two-wheeler on this bike, bushings are less of a problem, but with the painless two-wheeler method, even a child who can only manage the smallest sidewalk bike can learn to ride two wheels nearly effortlessly.
- Brakes. A few bikes have no ability to coast and no brakes. These can be easily recognized--Most will say "no brakes" on them, and when you pedal backwards, the back wheel goes backwards. Skip these.
Nice but not essential:
- Bearings in the crank and wheels: These will last longer and pedal easier, but won't interfere with learning to ride.
- Auxiliary handbrake: Hand brakes should not be the only brakes on a sidewalk bike, but can be useful for the transition to a two-wheeler, especially if there are hills in the training area.
- Air tires: Easier to pedal and faster, but can get flats. Foam tires don't go flat, but ride rougher and slower.
- Suspension. Looks neat, but should be counted the same as paint, or "motorcycle" look plastic. May cause extra mechanical problems, and makes the bike taller and less suited for a smaller child.
Luckily, bushings seem to have gone away in sidewalk bikes, or at least the ones with brakes. To tell if a bike has bushings in the headset: Look at a bigger department store bike, and look at the area where the handlebar stem connects--That is what a proper inexpensive headset should look like. The stem should go into a large nut that holds the fork to the frame. If the stem and handlebar are one piece, and a set screw holds them in, it probably has bushings. (I tried to find pictures of a good department store headset, but Google only had high-end bikes in the first few pages)
Cranks with bushings will generally have a roll pin instead of nuts holding them to the frame.
Cheaper bikes will have thick welded spokes, instead of standard thin tensioned spokes.