Having your young musicians play long tones while thinking about their embouchure will help them understand one of the reasons for practicing long tones. Alternatively, have them play long tones and focus on their air stream, air support, posture, pitch, etc. Choose a different aspect to concentrate on each day of the week. Ensure they understand the reasons behind each focus area. This approach will accelerate the benefits they gain from long tones (or any other fundamental exercises).