Their coordination skills are not developed enough to properly do this until they reach 8 years of age.
Start a program today to avoid later problems. You should laugh a lot and make it fun. First allow them to “brush” their own teeth. Finish by brushing your child’s teeth yourself . The best position for you (as well as conditioning them for the dentist) is to “be the dentist” and stand/kneel behind the child sitting or lying in a chair
Alternatively, have your child brush their favourite doll’s teeth before you brush theirs. Make it clear that you need to follow up by brushing their teeth correctly.
Use only a pea-size amount of toothpaste on their toothbrush as larger amounts tend to create excessive foam making it more difficult for your child to brush.
A low fluoridated “kids”toothpaste is recommended unless your child isn’t exposed to refined carbohydrates, sugar, honey, dried fruit or fruit juices more than once a month or so (i.e a truly “natural “diet….see Prevention/diet/Fluoride link for further information)