When my children were little, my husband accepted a job overseas. I was an only child, and my children were my parents only grandchildren, and, like you said, they were their life! Needless to say, they were devastated that we were leaving, but they put on a brave face, and gave us their blessing to go, saying they would visit for holidays, and we could speak on the 'phone regularly (this was long before Skype, etc.!). Frankly, being a very close knit family, I was very upset myself at the prospect of leaving them, but they never made me feel any guilt, and only wished us well.
Fast forward many years to us becoming grandparents. DH and I are still overseas, some 12,000 miles away from our daughter, who returned to the UK and remained there after university; and our DS, DiL and our only GC are in another country which is a 4 hour flight away from us.
We make the most of the times we get to spend together on holidays, and speak to each other regularly on skype/facetime/etc. Believe me, a 2 hour journey by car/train is absolutely nothing these days! How I wish I could just hop in the car after breakfast and be there by 10/11am!
I can totally understand your upset, and of course you are going to miss seeing them everyday, but your daughter has her own life to live, and if she has found a partner who treats her and the children well, and they love each other, then you really need to let her go with your blessing, and be happy that she's happy. The children will adapt to a new home/school. They are not yet of an age where they will have developed very close friendships with other children, and they will easily make friends in their new school. Much better for them to move now, rather than when they have 'best friends' that they don't want to leave behind.