This is why your dentist takes x-rays. You can see by the photo that the teeth look fine. But when the radiograph is taken you can see an interproximal cavity in-between the teeth on the right side of this upper bicuspid tooth. The bacteria that eat your teeth Streptococcus mutans are facultative gram negative anaerobic bacteria that cannot live in oxygen which is why they grow best in-between the teeth where there is no air containing oxygen and can only be cleaned with dental floss.
My doctor gave me this tip once: until your children can demonstrate to you that they can tie their own shoes, they're not ready to brush their own teeth. Kids simply don't have the dexterity yet to hit every tooth surface properly until they're able to tie their shoes, and who wants to deal with cavities being filled as a result of poor dental care?