NO, it's not enough if he wants to make progress.
Class-wise, once a week is ok. But s/he needs practice a lot outside class, maybe everyday. S/he has to do his/her homework, work on tactic problems, and play many games.
My suggested ratio for class:homework:tactic:games is 1:1:5:3. So if s/he takes one hour lesson, s/he should do one-hour homework, 5-hour tactics (spreading out the whole week), and play games for 3 hours. For a beginner, s/he can play 10-20 games in 3 hours. The total commitment is about 10 hours per week.
For a rated player, s/he should also play in weekend tournaments as many as possible.
If you follow this formula, you will see big difference in your child's games after 3 months.