It looked like on-again, off-again couple Shanna Moakler and Travis Barker would have a good summer, peacefully sharing their two children -- son Landon, 5, and daughter Alabama, 3. But today, when Travis was supposed to return the kids to their ex-beauty-queen mom at her family home in Rhode Island, that all fell apart when Travis met her with the kids, an insider tells Life & Style. "He started screaming and wouldn't leave the kids with her."

Last month Travis and Shanna approved changes to their custody agreement that let Travis take the kids on the road for a month while he was on tour with his band, Blink 182.

The fight escalated to the point that police were called to Shanna's home. "Officers were dispatched to that address about an hour ago," Barrington, R.I., Police Sgt. Tim Harrington told Life & Style around 5:45 p.m. when asked whether officers had responded to the Moakler address in the Providence suburb. Harrington would not discuss the nature or outcome of the call, but an insider tells Life & Style that cops had to make Travis go.

"Travis wouldn't leave because he wanted Shanna to let his nanny stay with her and the kids, but she refused," the insider says. "When Travis refused to go, Shanna called the police. They gave Travis a choice: Leave or get arrested. So he left. But he wasn't happy. He's already twittered about Shanna."

Travis' tweet leaves no doubt about his feelings toward his ex-wife: "My babies [sic] mama is a pile of sh*t."