First avoid Benedryl. It made my toddler so tired and grumpy he cried the whole time before takeoff and then fell asleep but it was not worth it for the first 30 min. Before takeoff. I had no choice that time since he had an allergic reaction to something he ate but I am not big on drugging my child just to pacify him. (personal opinion, not judging others) I travel all the time with my two boys. I have taken at least 2-3 plane rides each year since they were born. Yes, there have been moments where i thought to myself "i'm never doing this again." but for the most part it is always smooth sailing.
This is what I learnt:
1)always take day flights. A drowsy, sleepy kid equals a grumpy kid.
2) iPad with their favorite movie or game works best. I just play it with volume on and other passengers are just happy to have calm happy kids next to them. The plane motor drowns out the noise anyways. Mine watched a whole movie on the way there. He also read an interactive book: mickey's spooky Halloween which he loves!
3) special snacks that take a while to eat really does the trick. Package of Mini chocolate chip cookies or toddler cheese puffs for example are snacks that I rarely give so they are happy and take a while to finish. This keeps them busy for a good 15-20 min.
4) forget crayons, little cars, or anything that can roll off the trays and get lost under or in between the seats. They'll be having crying fits to find their toys and you'll spend half the flight on all fours looking for stuff. (not fun)
5) bring something like a new pop up book with animals for example. That really captures their attention.
6) make sure your diaper bag is easily accessible and pack 2-3 changes of clothes (1-2 for diaper messes and other for food or drink accident)
7) bring mini bottle of baby Tylenol on plane in case of sudden fever ( this happened to me on my last trip)
8) make sure your little one drinks on the way up and on the way down or sucks on a pacifier as to relieve ear pressure.
9) if your kid does start crying, don't worry. You will be surrounded by parents who understand. Never feel bad or guilty, you can't control what happens. Just concentrate on comforting your little one.
No point in worrying about it. Your child will most probably be fine. At that age they are curious and move around a lot but it doesn't mean that they'll be fussy on the plane. Just take it as it comes. Just remember, you'll land soon enough. the worse that can happen is that they'll cry or be fussy but that can happen in a restaurant or in public.
You can check out my blog
http://disneyplaning101.blogspot.ca/ for more tips on traveling to WDW with a baby/toddler
Good luck!