Non-elitest guide to department store bikes: Sidewalk bikes

The most important features are

1. 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.
   
2. 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.
3. 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:

1. Bearings in the crank and wheels: These will last longer and pedal easier, but won't interfere with learning to ride.
2. 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.
3. Air tires: Easier to pedal and faster, but can get flats. Foam tires don't go flat, but ride rougher and slower.

Silly stuff:

1. 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.