have your child eating foods high in selenium and you can give him a daily dose of Iodine as well. one couple who posts here give their boys, both school age i believe, 50mg each. they are now A students and sports leaders.
DQ just posted a good list of foods high in selenium just below this thread. you need it too, only selenomethionine, 800mcg. i suppose the kids be given 100mcg of that form. the supps i have seen come in 100mcg and 200mcg capsules.