Establishing good oral hygiene routines at a young age is the first step towards healthy teeth throughout life.
Start brushing twice a day from the very first tooth! Use a soft toothbrush and fluoride toothpaste. Use a small amount of fluoride toothpaste, the size of the child’s little finger nail. Help the child brush until the age of 10. A small child doesn’t have the manual skills to brush properly and can’t see the consequences of careless brushing. An older child may have to brush under your supervision.
More detailed recommendations for brushing children's teeth depending on age is available here.