She needs to start brushing on her own, but you must check her and go over her teeth again at least until age 5 or 6. Kids just don't have the dexterity and concentration to do the job that they need to do. that's why we put sealants on their 6 year molars. Those first permanent teeth are highly susceptible to cavities on the chewing surface because of their deep grooves. Kids just don't always get them as clean as they need to be. Ask your dentist or hygienist for some disclosing tablets. After she's brushed as well as she can, have her chew up the small red tablet, swish it around her mouth and then rinse and spit. Every spot that she's missed will stain pink, so you can easily see the areas that need more attention. Just be careful not to have her do that before she needs to go somewhere important, because it will make the lips and tongue a bright pink!