Is it safe for an 11 year old who weighs 81lbs to take benadryl (diphenhydramine) and melatonin every night as a sleep aid together?

Melatonin yes. : Melatonin yes. Benadryl (diphenhydramine) no. Why does he/she need so much to sleep well every night? I'd recommend further evaluation for why he/she can't fall asleep or stay asleep.

Yes: There are no drug interactions, however I would not recommend using the Benadryl (diphenhydramine) routinely. Practice good sleep hygiene. No TV, video games, texting etc within an hour of bedtime. Keep a regular routine. Avoid caffeine. Consider seeing a sleep specialist if persists ( or psychologist if you are concerned that stress or anxiety are the cause).

Yes and no: The answer to your question is yes it is safe. The next question that comes to my mind though, is why she needs a sleep aid every night at age 11. If she has a type of sleep initiation disorder,have behavioral interventions with her bedtime routine been tried, including no computer or LED screen time for two hours prior to bed? Will meditation and relaxation techniques help rather than chronic med