If your child enjoys riding and wants to do it regularly, you should buy them a pair of breeches. They have no troublesome seams and adapt well to the shape of the body. This prevents chafing and discomfort, which can be an issue with regular trousers.
You can choose between kids' Jodhpurs with an all-leather seat and those with knee patches. The leather seat or patches ensure better grip and stability in the saddle.
However, children's full seat breeches are considerably more expensive than those with knee patches. An alternative is a pair of children's breeches that grow with the child. They have an elastic, adjustable waistband. The length of the trouser legs can also be adjusted. They can be rolled up more or less and fastened with Velcro or buttons.