That is my biggest pet peeve about junior tennis - cheating. Sucks the joy out of tennis.
Once in a tournament a father of another kid was telling us how they got the same first round opponent as last tournament and from experience they already knew the cheating was wild. He said ball was landing on service line nearly in the middle of the court, and the opponent would call OUT. I thought he was making it up, who would call a ball like that out? Fast forward a few months, and one of my kids got to play that person. Let's just say that I am convinced it's parents teaching. On my kid's serve, 4 perfectly fine winners by my kid were called out (initially, instead of 40-0 and winning, cheating made 0-40 and loosing). It was unbelievable!! Other parents were sitting stoic and acting like all was good. By the time we got an official, my kid was an emotional mess and lost the match that should have been an easy win. Other parents walked away from bleachers without saying a word to us. Later on, we met another family who trains in the same club with cheater, and they confirmed it was their signature behavior - crazy cheating... What kinda parents allow, let alone teach, their kid to cheat? It was the best worst learning experience for us, and I only wish their was a system to submit complains about players. Maybe after a few negative reviews, they would suspend a player from tournaments for a certain period of time? I know there is no answer how to handle it, but it all starts with parents.