If the problem is on a few moves, look at the game to see whether they require that much time to think. Find a way to break the circuit. Get into the habit of checking the clock from time to time. Pick a time, such as 10 minutes at most for a move, stop there no matter what.
Analyze your kid's behavior during his/her games. Is s/he walking around too much? If yes, then suggest to sit down longer. There is absolutely no need to look at other people's games.
Try to use his/her opponent's time to think. Yes, the move is not there yet, but s/he can think against all possibilities. That's how s/he does his/her calculation.
Solve a lot of tactics problems. Build a correct thinking approach. Use a timer so s/he can learn to make judgement call when time is running out.
Play more blitz games, so s/he can move faster.