Or maybe your child isn't speaking standard English. Nagging your child to speak better won't help much.
Children imitate their parents.
Read aloud to your child from good literature so your child can hear proper English. Listen together to trained voices reading books or at the theater. Watch TV together, or listen to the radio, and discuss when the characters do and don't use standard English. Ask your child's teacher how you can help your child practice the standard speech taught at school. If you suspect your child has speech problem, seek professional help.