I have constant sinus issues, so I always take sudafed when I'm flying. Not sure if this is an option for kids. (I get the behind the counter kind.)
I get stabbing pain in my ears during the descent and lose some hearing for a few days often when I fly. It's worse if I have multiple flights in a short time frame (layover or a short trip). My doctor also suggested afrin but I keep forgetting to pack it.
Talk to their doctor, but allergy meds should help. Benadryl and Zyrtec are both safe (and Zyrtec is non-sedating). I wouldn't do a decongestant like Sudafed until talking to the doctor, but have myself taken Afrin just before a flight. I get the same issues when flying (I have chronic allergy/sinus problems) and I ALWAYS forget gum and end up bumming one from a flight attendant.
My ENT says everyone should take Sudafed and Afrin when flying - I’d ask the pedi about the sudafed obviously (we have it to DS1 starting when he was 11 even though it’s ages 12 and over, but pedi ok’d and he was over 100 pounds etc)