Grandparents want to move away?

Two years ago my parents moved across the province to be closer to our family. This was a good move for them in many ways and allowed my dad to retire. Now, however, they want to move back. According to my parents, the gr is greener back where they came from. So, they are planning on returning and leaving our small family of five (two adults and three small children) on our own. My kids are devastated and heartbroken. I am angry that they are moving back across the province, away from the family they so wanted to be near. We've worked hard to help them settle and I feel it was all in vain. I think their reasons for moving are selfish. Don't most grandparents want to be near their grandkids and extended family?? Should I give them my blessing to leave? Do you think my anger and hurt is justified?