Britney thumbed her nose as a custody agreement with soon-to-be ex-hubby Kevin Federline, by taking their two children on a jaunt to Hawaii – and he’s now vowing to take the children away from her. Kevin hit the roof when he learned Britney was wallowing in the Pacific sun without a nanny, says sources. Britney’s custody agreement with Kevin states that she can’t take the boys out of L.A. County without his consent and without a nanny. An insider reveals, “For two days Kevin was desperate to find his children.” Kevin, worried over Brit’s drinking drug use, and wild behavior, had refused to give his OK to the Hawaii trip. Kevin bombarded her with phone calls, but she refused to answer. When Kevin checked the hotel Britney has used on previous Hawaiian vacations, she wasn’t there. According to an insider, when Britney finally answered her cell phone she infuriated Kevin even more when she told him she was indeed in Hawaii, sitting poolside sipping a drink. Kevin could barely control his temper. He issued an ultimatum: Get on a plane with the kids by noon on June 8 and head back to L.A. Britney deliberately missed the noon deadline, but did get on a plane later in the day. When Kevin finally got to see his boys, they were exhausted. Now Kevin is determined to gain full custody of his children by going back to court to argue that Brit is an unfit mother, with the trip to Hawaii just the most recent incident in the loopy behavior that she’s been displaying. “Kevin is upset that Britney’s partying hasn’t stopped. She’s drinking heavily, and Kevin believes she’s also doing drugs,” said the insider. Kevin says, “If the court awards full custody to him and Britney loses her children, she’ll have no one to blame but herself.”