The usual recommended wear time for children's removable appliances (plates) is 22 hours a day. They are taken out only for eating, brushing teeth, sport and singing. If the appliance is worn only at night, it often drops slightly away from the teeth, because our muscles are relaxed while we sleep, and over time it stops fitting. Results can be seen even with night-time wear alone, but because of the effect I have just described, wearing it that way isn't recommended. The minimum wear time for an appliance is 14–16 hours, which means all of the child's free time after school and through the night.
This answer is for general information. Your own case needs an examination by an orthodontist.