Lots of peds will recommend Benedryl (an antihistamine) when flying, but it doesn't really help ear pain directly unless you get the formula that also contains pseudoephedrine (a decongestant). What Benedryl DOES do (for most children) is sedate them so they will sleep through the flight; most peds seem to think that unconsciousness is the preferred state for a small child on a plane. If the pressure isn't too severe, sleeping through takeoff and landing will allow a child to not notice mild pain until it has already passed. BE AWARE that some children react adversely to Benedryl and become totally wired; never do Benedryl if you haven't tried it first at home.
I have Meniere's disease, and I have all sorts of problems with ear pain. The key to preventing it is a good decongestant, and I find that one that has pseudoephedrine AND guiafenesin will be the most effective. If you have serious issues, a Rx steroid nasal spray is a good backup to carry in case the oral decongestant does not work. (I personally won't touch OTC nasal sprays, they are highly addictive.)
You can also ask a Dr. for Auralgan analgesic drops for the ears, which work instantly, but many pediatricians will not prescribe them because using them on children can mask the pain of a burst eardrum. If that happens the pain is excruciating, but it warrants emergency medical attention; they don't want you not noticing that it happened. Canadians can buy Auralgan over the counter, the lucky pups.
BTW, chewing doesn't do a darn thing for pressure, it's the swallowing that helps open the Eustachian tubes. (When you chew gum or suck on hard candy, your mouth produces saliva, which you swallow.) One of the best things you can do for everyone in your party is to carry bottled water or water in sippy cups; drink continously from them during takeoff and landing for best results. (It helps prevent dehydration, too.)
Earplanes are great if they work for you, but they don't work for everyone, and lots of young children will not tolerate having them placed in their ears.