What is the best OTC sleep med to give a child with adhd?

Adhd with insomnia. Depending on age and what medications your child is on for adhd, some options to discuss with his doctor could be clonidine, intuniv, kapvay, tenex (guanfacine).
Melatonin. Try melatonin to help with sleep in adhd. If still not sleeping, see your child's pediatrician about a trial of clonidine.
None. You didn't say your child's age, but no otc medication should be routinely given to a kid, especially under the age of 12. Melatonin isn't good to give, it's a hormone and is best used safely for adults. Benedryl can cause cognitive impairment with chronic use in kids. For my patients, i often have prescribed Guanfacine (tenex), one half to one milligram. Ask the doctor.