I got questions from a few of you regarding what magnesium supplement to give children and also about the laxative effects of magnesium so I thought I would answer those questions. You should let your pediatrician know what your are doing as well. Anyway...
Not all magnesium supplements are created equal. Magnesium oxide is what you will find in most vitamins but this type of magnesium is very poorly absorbed. Magnesium Citrate is well absorbed but is not available in a children's formula and can cause violent diarrhea. The best solution then is Magnesium Hydroxide. This is actually a laxative as well but it is much more gentle and if you give your child one half of a 400 mg pill a day this should cause very few gastrointestinal symptoms and should be an ideal supplementation amount.
Amazon sells the Magnesium Hydroxide in a formula that is made by Fleet and is called Pedia-Lax Magnesium Hydroxide. You should be able to buy this at Walgreens, CVS or Walmart as well. It costs about $20.00 for thirty tablets but you will be giving your child only half so this should last you for 2 months. It is a watermelon flavored, chewable tablet and most kids like the taste.
There are multiple adult supplements available that contain a combination of calcium, vitamin D and magnesium. The most affordable is probably Solaray Cal-Mag Citrate. If at any point you or your child develops diarrhea or loose stools while on the supplements, you will need to decrease the dose.
The ratio of calcium and Vitamin D to magnesium matters because the absorption of the vitamins and minerals is altered by the ratio of one to the other. The Soloray product has the appropriate ration of vitamins and minerals to optimize absorption. It is important to continue the multivitamins as well as the B vitamins and vitamin C are also important in making certain that the body utilizes the magnesium that it has properly.
Let me know if you have any other questions.