I don't know yet - so far, they call
Franciszek Franio, but they'll probably progress into Franek once he gets older (I think he's 3 now). As much as I like
Fryderyk, I agree that it doesn't have many good nicknames.
masculine list:
https://www.behindthename.com/pnl/191050/124079feminine list:
https://www.behindthename.com/pnl/191050/124080