I'm going to tell my dad that if he wants to take one of my kids to go camping, he has to take both
Does he really have to take both? Why's that exactly? I'm curious now.
Because I know how to get my way, I learned it through my dad.
Definitely it is necessary to be fair with your kids, but your dad doesn't have that burden with them so I think that it is not necessary for you to demand that he brings both of the kids with him. Your dad may find it very difficult to take care of both kids, and may not really enjoy the camping anymore. How about you ask him to take the other kid on a later date, or bring the kid with you instead?
Nah I'm just being mean.
Are you sure you can't just take them both on the trip yourself? I think your kids would love that.