Our rule of thumb on brushing is that until kids can tie their own shoes, mom or dad need to help brush teeth because they do not have the dexterity developed yet to properly clean all surfaces of their teeth. Up to this age (around age 6), a manual toothbrush is usually sufficient.
After age 6, you may find the issue is your kid just is unenthused and “forgets” to brush. One way to make this healthy habit easier and more fun is to engage your child with an electric toothbrush and a brushing/coaching app. We really like Phillips Kid’s Sonicare and sell them in office. The Philips Sonicare For Kids app synchs with your kid’s sonic toothbrush to help them learn about oral care through play. Your kid’s brushing coach challenges them to brush for longer and better. It has a timer and pacer to ensure proper brushing. You can even check the app’s calendar to make sure they brush twice a day like they should! Disney also has a brushing app that you can use even with a manual toothbrush.