Does he want to go into nursing because he thinks he would enjoy it, or is he just picking something that is in more demand now?
If he is passionate about nursing and doesn't feel satisfaction from his current job, I would say to make the move now. Your kids are still young and changing preschools is a lot easier than changing elementary schools or middle schools.
Honestly, as much as you and your kids might like their friends and teachers, will it REALLy matter 30 yrs from now? Or would having your dh find a fufilling and stable work environment make more of a difference? I think the answers to these questions will vary for each individual, and there is no right or wrong answer, so you need to think about your own families priorities. Honestly, I had some teachers and friends I really liked when I was a child, but I probably would have found new teachers and friends I liked too if we had moved. I grew up fairly poor, so if my mom had found a fufilling stable career back then, it probably would have a more positive influence on our lives now.
I guess what I'm saying is try to imagine the effects each choice will have on your family several years from now, not just the immediate effects of sadness of leaving, etc.