If your child has already played some tournaments, has some experience with how tournaments are going, and is comfortable with the competition, s/he should try to play in some long time-control tournaments. Most of them are adult tournaments, which mean that s/he will possibly play against some adults. It's great practice for him/her.
Short time-control tournaments, such as G30, or G45, are good to keep you familiar with the game, and provide you more chance to practice because you can play 3 to 5 games in a day. You can at most play two per day in long time-control tournaments. But the problem with short one is that you can't think about complex strategies and long plan. Endgames will also be rushed through because you most likely don't have time at that stage. Because of time pressure and time trouble, most players have instalbe performance in short time-control tournaments.
Do not ask the question, "is my child ready for long time-control tournaments?" My daughter's third tournament "Columbus Open" is a long time-control tournament. She was less than 700 and played everyone 300 to 600 points above her. Losing 4 but luckily winning 1, she picked up 100 points while gaining great experience.