I totally agree with the others. Just do it.

We moved from NSW to the NT. We had already moved away about 600km so they were used to not seeing the kids.
Totally agree with Audax about it becoming normal for the kids. I never had any grandparents and I never knew any different, whereas DH's parents and grandparents have only ever moved within a 15km radius their whole lives and thought we were doing it to punish them. Hell no. The only people living our lives are us and we wanted to do it to the fullest for us.
Relationships are two sided and if your families want to maintain good relationships, then they will. It's that simple.