K.C.
M.,
Ditto what the others have said. My 3 1/2 yo daughter has flown to England three times. I did use Benadryl once, but mostly it's been a case of similating your bedtime routine from home. Her own carseat (if you're allowed to use it) is great, because she's used to it in your car (two was too early for mine not to be strapped in:-). A two hour domestic flight taught me that one!) You get on board, have dinner, change her into her jammies, give her some milk (they'll warm it if you ask) set your watch forward, and read a book or two. Make sure she has her favorite blankie. If you limit her afternoon nap, she'll be tired anyway, and the motion and noise will entice her to sleep. You might want to put her next to the window so you can get in and out to the bathroom easily. Walk with her up and down the aisles if you need to, and definitely use anytime you have before the flight to let her run. I didn't realize bwi had a play area (I'll look for it now), but they do have some ramps we used on the last flight which we let her run up and down on before the flight. Make sure you take snacks she likes with you. If she's into stickers, they're great for keeping her occupied for a while!!
Good luck, K.
ps-I find explainïng the steps as we go helpful to my daughter..."we're going to get on the plane, get in your carseat, have dinner, etc..." Then there are no surprises.