Fun with DNA

I have two children, who we can call "Sandy" and "Chris," and four grandkids, 2 boys and 2 girls.

Following my older grandson’s Y-DNA brings us to Germany, maybe around Bremen. My younger grandson’s Y-DNA goes back to Poland, near Warsaw perhaps. Two of my grandkids’ mtDNA goes to Carinthia, Austria. The other grandkids’ mtDNA reaches to Northeastern Ukraine.

Both of my children had the same father and neither had children by more than one partner. Do I have two sons, two daughters, or one of each?