When flying with a toddler or baby and you need to change their diaper you can change the diaper in the plane bathroom.
In the planes bathroom there will be a trash can where you can dispose of the babies or toddlers dirty or wet diaper there.
I've always went to the toilet with my kids when they were in diapers and we were flying and they needed a diaper change.
I changed the diapers of my kids in the plane bathroom and then threw the dirty or wet diaper into the trash can on the plane.
Do not change diapers of a toddler or baby out in the open especially if it's poop as it can spread bacteria and nobody wants to or needs to see your babies poop or private area.
Always change the baby in the bathroom.