The main difference between WTF and ITF is WTF is South Korean and ITF is North Korean. The forms are different, kicks are the same. Attitude in ITF may be geared more towards self-defense and WTF is certainly geared more towards sport sparring in most dojos.
Actually to better answer your actual concern, would it be difficult for your kids to transition:
This depends. The new things that they would have to learn in WTF, such as forms and one step sparring, will be easy to pick up. Things that they have already learned, like kicking and blocking, may be difficult if the WTF instructor teaches those moves with a different variation. Its always harder to change a habit. (Or the teacher will see it as something to fix) Best bet is to go to the WTF school/s you're interested in, talk to the instructor, see if they can take a class, and see if it fits. A good instructor will be able to tweak things that aren't consistent to his style over time.