Can I coach my child during a match?

Only at specific times. In USTA sanctioned play, coaching is not allowed between points, at changes of ends, at the 2-minute set break, during any warm-up, or during toilet, medical or bleeding breaks (USTA Comment 30.2; Reg. IV.C.10).

Coaching is allowed:

• During a 3-minute or 10-minute rest period between sets. In junior events this happens on court or in a designated area near the court where an official can time it. It may be by phone, but the phone must be turned off when the rest ends (FAC Comment III.E-2).
• During a suspension of play, such as for rain, when the players leave the playing area.

Your child may read notes prepared before the match, but can't receive information on a phone or smartwatch on court (Comment 30.1).

Audible or visible coaching at other times is a code violation against your child (point, then game, then default).