My Grandchildren Are Visiting From Out Of State. Their Mother

My grandchildren are visiting from out of state. Their mother as both of them on 15mg of melatonin. My granddaughter is 5 years old and my grandson is 12 years old. I have so far cut them back to one 3mg pill at night; however, I still feel this is too much. Oh, sorry, my grandson has adhd and has not been on his medication for awhile.
What would you recommend that the proper dosage for each to be. Neither child has seen a doctor, the original 15mg's was their mother's idea. What should they be getting? And, can melatonin cause growth problems in either child.