M.L.
With all the posts on this site from people who hate their in-laws or feel oppressed by their own parents, your situation is actually like a breath of fresh air!
Of course, you would like them to stay. And you don't know that they won't. They may be enjoying life where they are, but a job is a job. You know this, of course. If your dad gets this job, you'll want to rejoice with your parents and make yourself available to help them as much as you can.
And actually, those very actions will help your feelings. It can sometimes be harder to think about what *might* happen than when the event takes place. Try not to play the what-iffies.
My older son was jobless for quite a while, not many years ago. The job he finally found had good pay and good benefits, but it entailed a lot of traveling. In fact, he's become so good at the job that the traveling has increased. It's hard on him and it's hard on my DIL (as well as the children), but they have risen to the occasion and do what they need to do to make it work AND keep the family happy. I'm quite proud of them.
One thing you can do, if your parents move, is to start planning a trip there. And technology has advantages. If your folks are far away, you can still communicate sort of as if you lived next door. If you get Skype, your children can see their grandparents every day.