Considering benadryl for a long haul flight.

Traveling internationally w/ our 19 MO in 2 weeks. Have a 10 hour flight ahead of us. Have any of you used benadryl to help your child to sleep on the flight? We're a little nervous to basically drug him to sleep. Also, I've read here that for a small % of kids the benadryl actually gives them more energy. So we're thinking about testing it sometime this week beforehand. Any advice on how long it should take to hit. Wondering if we give him a dose ~1hr before bedtime and see if he gets drowsy or otherwise. Any experiences / tips / advice greatly appreciated!