Don't use the type, where an 'arm' attaches from the seat post to a normal kids bike, poor ride quality and control. We use an alloy frame (lighter), single speed, foot brake, rear wheel only, a proper joint/arm attaches to the seat post - it's a fold up as well. Ride is sturdy with some small movement. About $135.00 Ray's Preston. This is on an MTB, rides very well. Have done up to 40kms distance (and speed
) no worries.
The child still gets a ride experience, just teach them to always hang on, otherwise if off the rear, happen to us once!