I don't like using Benadryl on my kids. We tried it on Dylan, different variations, and they stimulated him. The fast acting tablets that dissolve on the tongue, OMG, they made him NUTS. If you read the prescribing info, I think it even says it can cause hyperactivity or overstimulation or something to that nature.
I highly recommend Melatonin over Benadryl. Melatonin is natural. I give it to Dylan, as needed, 3 mg one hour before bed. He is 12, 150 lbs. My youngest is ADHD, Combined on Concerta. He has a very hard time falling asleep. He takes a 3 mg. tablet, too. He's 10, 50 lbs. You can always cut this in half, too.
G'luck.