Is it safe to give children benadryl (diphenhydramine) to sleep when they're 10yo?

Yes if used seldomly: Plain Benadryl (diphenhydramine) is safe to use in children to treat allergy symptoms such as hives or other itchy rashes, itchy / watery eyes, and itchy / sneezy / runny noses. Benadryl (diphenhydramine) makes most kids drowsy, so it is safe for occasional use as a sleep aid, such as when cold symptoms prevent one from falling asleep. Children should not need sleep aids on a regular basis, unless prescribed by a pediatrician.

Sleep: No. Benadryl (diphenhydramine) or Diphenhydramine can stick to the brain for 3 days impairing motor and skilled performance, judgment and memory etc.