I strongly suggest l-theanine. It's an OTC amino acid derived from the leaf of the green tea plant.
Good for anxiety, good for sleep, inexpensive, NO side effects, and comes in child size doses.
A 9-year-old child I know, who takes melatonin occasionally, did get Covid-19, but had no symptoms. Was only tested, due to her household had it, and her mother is a nurse.
A 9-year-old child who takes melatonin?..............why?..............