First of all, the weight of the bike is essential. A good children's bike should be able to stay easily under their control when stationary, in motion, but also when braking. The ideal bike for your child should therefore be light and not exceed 30 to 40% of its weight.
The geometry of the frame must be adapted so that your child is in a stable and comfortable position. Sitting upright will allow the child to look straight ahead. A child must be able to get off or step over the frame very easily in all situations, not have his legs apart and be able to put his feet on the ground in all circumstances. For this, your child needs a bike with very low center of gravity, a suitable frame shape and a narrow crankset, like on a BEMOOV bike.
The length of the bike, as well as the angles formed at the level of the fork or the seat post, have been designed to favor stability, comfort, and safety.