Jay-Z and Beyoncé took a crash course in parenting Sunday when the expecting couple mixed with brownstone Brooklyn families at a kiddie birthday bash. “It was a party for Beyoncé’s niece,” said a guest at Carroll Park, blocks from Jay-Z’s favorite pizza place, Lucali. The Brooklyn-bred rapper has made offers on homes nearby, residents said. Beyoncé, meanwhile, offered to sign a girl’s cast. But her security shooed away at least one parent looking for a photo.