Is it safe for my 12 year old to take 10,000 mcg biotin for hair growth once a month .

Hmmm....: Biotin deficiency is rare, but can occur in persons with genetic mutations, on long-term antibiotics or anti-convulsants, or who eat a lot of raw egg whites. There is no good evidence that this easily available, over-the-counter supplement is the magic potion to grow hair or prevent hair loss. Biotin is safe to take, but 10000 mcg per day is over 60 times the usual dose, so is not advisable.