As a failsafe if all preventative measures doesn't work, ask your pediatrician to give you a scrip for Auralgan analgesic ear drops or the generic form of them. Be prepared for a refusal, because the drops can mask such conditions as ear infections and/or burst eardrums, but they are invaluable in a pressure-pain situation. What I did was essentially sign in blood that I would not give more than one contiguous dose to my child, so that if there was an underlying problem causing the pain, it would become apparent after the initial dose wore off.
One of the few other things that normally helps once pain starts is the steam cup trick. If you need to resort to this, you must ask for these before the FAs strap in for landing. What you do is ask the FA for two coffee cups stuffed with paper napkins, with a bit of boiling water poured into them; they are familiar with these kinds of requests. What you will do with the cups is hold them firmly over the child's ears like earmuffs, because the steam contained in the cups will help to equalize the pressure in the child's ears. IT IS ESSENTIAL THAT BEFORE PUTTING THE CUPS OVER THE CHILD'S EARS, YOU NEED TO HOLD THE CUPS UPSIDE-DOWN FOR A FEW MOMENTS TO MAKE SURE THAT NO EXCESS HOT WATER WILL DRIP OUT TO SCALD THE CHILD.