Abigail gave me the clue. As soon as she heard the tune she said “That’s Nanny Nanny Boo Boo”. Simon disagreed -- “No, it's Nanny Nanny Poo Poo.” So I had something I could Google -- I just tried Nanny Nanny -- and yes, there it is, part of an entry that's all about "Taunting."
Now I know how to sing “You can’t catch me” in Croatian, and I am also in a position to tell you that the crotch-grab is used almost exclusively by males. As for the gesture below, it goes back thousands of years, and was referred to by the Ancient Romans as the digitis inpudicus.
p.s. -- Happy April's Fool's Day -- it started snowing more than an hour ago and it's still at it.