Speak correctly to model good grammar. When an error is made by the child, restate it properly as you continue the conversation so the child hears good grammar.
Use mentor sentences from books you are reading to teach grammar. Here is what I have developed. Replace the mentor sentences with sentences from books that you are reading. For 1st or 2nd graders, think of this as exposure to grammar. Have the young child say sentences orally. Ask them to write their own sentences, modeled after mentor sentences, in 3rd and 4th grade.