Training wheels have long been the best way for children to learn to ride their first bike.
It is often an advantage to progress from a trike first then onto a bike so the fundamentals of steering and pedalling have been learnt in a safer environment where the child balance is taken care of by the trike.
Often the training wheels should be just that "training" devices used to assist the child to the next step in riding a bike without them. Often through poor set up and incorrect adjustment they can also be the cause of problems in learning to ride.
As most breaking is carried out on a child's bike by the rear wheel it is essential for the rear wheel to have full contact with the road surface. Each training wheel should allow the bike to lean over slightly each side this helps in two ways, promotes the learning of balance not being totally dependent on the training wheels as well as along the rear wheel to maintain full contact with the ground.
As the child has more trips out on their bike the training wheels need to be gradually raised up a little further from time to time to make the bike less dependent on the wheels and the child learns to compensate and with practise learns the art of balance automatically.
A big mistake a lot of parents make is never touching the training wheels once attached and then removing them when they think the child should be ready to ride without them, instead of preparing and training their child for the time when the wheels come off. This can result in a tougher longer learning curve at adapting to a bike without them and often tears and feelings of inability which we don't want when teaching something that should be all about fun and family.