You could start out with putting on the pull-ups in the airport restroom if the girls are agreeable to that. If the 12 hour flight is a single non-stop flight, then changing during the flight may be necessary. If it is several shorter flights, then you may be able to get by with changes in airport terminals during layovers. Either way you will need to carry spare pull-ups with you. Airplane washrooms are cramped, but you can change in one if you need to. Just wear clothing that can be easily removed in tight spaces. I have seen kids change from their daywear into pajamas in them during long night flights. Carrying a small bag to the washroom would not be noticed. People carry all sorts of toiletries on overnight flights, like toothbrush, battery razors, tampons, makeup, etc. One alternative might be to wear two pull-ups if it isn’t too bulky. If the inner one gets wet it can be ripped off leaving the outer one in place without undressing completely.