Honestly, everytime they go on a trip you will have to change them over and back again when they get home. Oh an make sure you do it after hours so the dont get cheap service until the leave.

Do you REALLY want to do that kind of work?

Only other way is to have them pay for the extra charges.
After one time they will turn off the data on their phone.
Well maybe after the second time, one user here is on his 3rd charge