Good for you! I offered to pay for my oldest granddaughter's education in a Christian school but they didn't want her to go. How foolish. Fortunately she's a bright young lady, well thought of, and not a problem, but what about the kids who are? My sympathies go out to them.
How old is your granddaughter? I find it hard to believe that parents with very much current knowledge of the public schools would prefer any of them to a private Christian school in most circumstances, no matter their religious convictions or lack thereof. If someone had made me that offer for my kids, I would have taken it in a heartbeat for my youngest. My older girl was in advanced classes (which are both better scholastically, and tend to get the better teachers), and played in the school's top band (which wins awards all over the country), so I'm not sure she would have wanted to leave all that. But, I think my youngest would have been MUCH better off.
However, the way to do it is to start them there from the very beginning, in kindergarten. After a certain age (different for different kids) it gets very hard to get them to leave their friends.