What I did during my girl's first flight at 8 mths was lots of toys and things to engaged her and snacks. Give them something to suck to soothe the pressure so if you're still breastfeeding, you may want to do that, or give them a pacifier. You may also want to talk to her prior to the flight and prepare her what to expect. They may be young but don't underestimate them.
Let her drink milk or eat snacks or suck lollipop or sweets just before it takes off... In the air usually no issue, just prepare some stuff to keep her occupied, be it iPad, toys, whatever that keeps her still for some time. You can try to baby wear her to make her sleep also since it's such a long flight...
Let her suck on something (sweet, puffs, pacifier) just before the plane starts to fly off or land... Or drink milk from the bottle... This sucking motion helps to reduce the air pressure in the ears.
Let her have her favorite with her, like milk, pacifier, toys and snacks
Let her suck on something to not have ear pressure
Let them suck sth to decrease the ear pressure