EVEN celebrities can have trouble finding a babysitter. Mira Sorvino was stuck bringing her kids along for the New Year's Eve festivities when she hosted an event at Nikki Beach Midtown. Our spy said she showed up at 11 p.m. with her 2-year-old daughter, Mattea Angel, and 6-month-old son, Johnny. It didn't keep her from having a good time, or a cocktail. She did, however, cover her kids' ears with headphones, said a spy, "to protect them from the noise." When Sorvino hit the door at 1:30 a.m., Mattea "threw a tantrum," we're told. "She didn't want to leave the party