That is a scary, scary quote!
Do Calvinists believe this? I thought "God has no grandchildren."
I'm sure someone has said it here before but I will say it again: Not all Calvinists (with or without quotes) believe in all of the teachings of Calvin or the WCF. I certainly don't, though I would be called a Calvinist. I prefer Grace Christian, or believer in the doctrines of grace.
The quote of Calvin is from his Institute (an overall excellent book in most points) and is based in his erroneous adoption, IMO, of paedobaptism.
It irks me to see that many think of "Calvinists" as all getting their marching orders from a man or the Westminster Confession and not from Scripture (where I get mine). I read and respect Calvin, Luther, Owen, Gill, Spurgeon, Gill, MacArthur, etc but in each of these good men I hav seen deviation on some point or other.