My cousin Ariyonna is seven years old, and unfortunately she doesn't speak very well. It's not that she can't speak, she just doesn't do it correctly. Is there a way to teach her, keep her attention, and make sure she remembers?
I wouldn't start any grammar drills. Instead, read stories and books aloud to her to give her the best models of the English language. Read simple plays with each of you (and friends) taking roles. It will expand her vocabulary as a bonus. Get her talking on outings or during movies by asking questions (play dumb) that gives her practice in sentence structure. If she errors in this, just echo what she has said correctly, but without correcting her; confirm what she has said. Language, written or spoken, is acquired with usage and practice, but must always be interesting or relevant to stay with the student.