Leighton Meester might end up having cereal for dinner tonight, and that’s OK with her. “It’s actually pretty good,” the poly-hyphenate actor, singer, songwriter, and Broadway veteran laughs. Her impressive career reinventions aren’t what is keeping Leighton busier than busy—she has those under control. It was when her daughter—with actor husband Adam Brody—was born, three years ago, that the juggle got real and proper dinners became more sparse. “I feel that I am really lucky, and yet, still, I stepped into the hardest job in the world: being a mom,” she says. In fact, there’s a scene in Single Parents, her new ABC sitcom, about a tribe of Angelenos trying to keep it together raising first graders solo, that sums up where Leighton is with her life right now:…