A child should see a dentist every 6 months as a preventative measure.
When you take your child to the dentist the child's pediatric dentist will recommend another dental checkup visit 6 months after the first dental checkup and then every 6 months after that.
Make sure your child is brushing their teeth and flossing to keep their teeth clean and free of bacteria, food etc.
This helps prevent cavities, tooth decay, gum disease etc.
A child should be supervised while brushing their teeth until they are at least 6 to 7 years old.
After the child gets to 6 or 7 years old and above they can safely brush their teeth on their own.
Although if your kid is like mine I have to make sure he brushes his teeth even though he's 8 years old I have to sometimes brush his teeth for him because sometimes he will never brush his teeth unless I force him.
My kids all see the dentist for cleanings and a dental checkup every 6 months unless they have another dental issue such as a tooth infection then they see the dentist when needed.