Invite them to your house. If they can't afford it, buy air tickets for them. Do it during your work time. After that, feel free to take your own vacations.

Another alternative would be: spend a long weekend with them, then on to your vacation destination.

I assume, your vacation is with the children? Think of all the things your children are deprived of.

Do you need a reason not to see your parents? How about this: mom, dad, we're going on vacation, would you baby-sit my children for a week? No? Sorry, in this case, we're taking them to Yellowstone this year!