Re: Solution to the problem
Babies and toddlers can equalize pressure in their ears when they suckle or are given something to drink - anything that promotes swallowing. Do that during takeoff roll and they're fine through to cruising altitude.
Our child traveled the equivalent of two round the world trips before he was two - a combination of multiple transpacifics and US transcons. He has no trouble with it and quite enjoys flying, with his trusty bag of toys and ipad, and his preferred window seat to play in calmly, and makes a show of swallowing right after takeoff too.