I would bring a few pull-ups in a carry on bag. The girls wouldn’t have to wear them all the time but could put one on in the plane’s washroom if they wanted to go to sleep. You could also carry a spare in case one of them needed a change. I wouldn’t worry too much about airport security. They do sometimes search carry on bags but they are very discreet and would not pull something like that out where other passengers could see it. I do a lot of flying and always carry one diaper in my carry on bag in case my checked bag goes astray. No one has ever said anything about it or pulled it or anything else out of the bag.