My 7 year old son complains of cramp behind the knee most nights. i give him magnesium tablets but these don't seem to help. what else could i do??

Night cramps: Night cramps in kids have various etiologies (reasons). I would start with 3 simple things if the magnesium isn't working. First, do calf stretches every night before bed. Make it part of his bedtime routine. Second, make sure he is taking plenty of water daily. Divide his weight by 2 and that's how many of ounces of water to drink each day. Finally, try increasing potassium - bananas, pineapple.