My kids currently train under UKTF, which follows ITF guidelines and techniques. Will this cause problems if they move to a WTF trainer? They aren't quite at black belt yet, so I'm hoping that if they do transfer it won't be too much of a problem.
It depends on your kids, really. It will take them time to get familiar with the WTF style of Taekwon-do. I was a WTF kid before joining the ITF as an adult. I was a red belt then, now a 2nd Dan in ITF. I did not transfer my rank and worked my way from white belt in ITF. The transition was OK for me, because I went in with an open mind.
Are there differences in technique or style that would make this a difficult transition?
Yes there are differences in technique. For instance in ITF style Taekwon-do there is the sine wave technique to utilize body potential in strikes and blocks. In WTF Taekwon-do there is no sine wave, which you may say is more similar (than ITF) to traditional karate.
This may not be a technique but the spelling of Taekwon-do reflects your side of the coin. ITF spells it Taekwon-do and WTF spells it Taekwondo.