Getting them to suck on a lolly or bottle/ breast (depending on age) is the most effective way I know of.

Gum or having a bottle of water, milk or your breast handy 🙂

I make sure I have lollipops as well as gum. It is important the jaw is in constant motion, yawning helps as well.
You can also use nasal sprays which help, but I am not sure how old your child is.

Moms have given great advice as usual and these are also helpful.
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Lollies with Eucalyptus are very good. Inhaling Eycalyptus on special wipes or tissues helps too. A few years ago we were on a plane that was definitely incorrectly pressurised. Everybody was having problems, even those who were sucking lollies.

I’ve heard that sucking one lollipop is very useful!

Breastfeeding OR bottle on take off landing. NB: don’t start till the Stewarts take their seats as taxi-ing can take a long time! I also I took on loads of little snacks, such as sultanas, yogurt drops, Heinz nutrios and my 15 month old snacked, meaning he was chewing and therefore popping his ears, I had no issues! Good luck

The travel agent told me to have them chew something like lollies or twisties on take off and landing