Trainer wheels can help a child develop riding confidence, but they should be used for as short a time as possible, so the child isn't held back from learning to ride properly.
As the child develops riding confidence, try raising the trainers up a little further from the ground so that the rider has a chance to ride without using them. When you see your child riding properly (by balancing rather than by leaning on the trainer wheels), it's probably about time to take the next step and remove the trainers, but be prepared for some resistance, and also be prepared to work with your child a while (on some grass somewhere) to get them riding without trainers.
Trainer wheels come supplied with all new 12" and 16" bicycles, and are available to suit 20" bikes also. They don't last long, so if your child needs to use them for a fair while, you could be buying a few sets. However, if the trainers are starting to be destroyed, that could be a sign that they might not be needed any more.