More than 11 years after Britney Spears and Kevin Federline called it quits, they are still at each other's throats — and it's all because of their kids! The ongoing custody battle for their sons, Sean, 12, and Jayden, 11, is just starting to heat up and Britney is "devastated" over it. The popstar was served a subpoena from Kevin's attorneys in June and didn't exactly handle the news well.

"Britney immediately started freaking out when she got subpoenaed," an insider told Life & Style. "She's trying to stay strong, but she's been panicking." And she has a right to be upset. Kevin is asking for a lot more money in child support — like, three times more money than Britney is paying him now. Plus, he wants more than just the child support.

"He also wants back pay and all his attorneys' fees compensated, and he's threatening to keep dragging it all out until he gets his payout," the insider said. "Britney and Kevin are only communicating through their lawyers at this point." Clearly, things are getting nasty.

Back in May, Kevin filed documents stating why he wants more child support and we don't exactly feel bad for him, to be honest. "My income has changed significantly since the child support was last modified in approximately 2008 because I no longer earn a six-figure income like I did in 2008. I am no longer able to perform as a dancer due to my age, and I have not been as successful in putting out new music as I was in 2008," he stated in his declaration. "I am simply less of a 'name' and less in demand than I was in 2008, which has negatively affected my income."

Kevin has been receiving $20,000 a month from Britney to care for their two sons ever since he was granted primary custody in 2007, following her mental breakdown. But, that amount was based on her income at the time he received custody. Now that Britney is thriving in her career again, Kevin thinks he's entitled to more dough.

"She's trying to stay positive," the insider explained. "She's in a healthy place these days, but friends are fearful this stress could set her off." Fingers crossed Kevin isn't the reason to make her spiral, again.