Kevin Federline feared for his sons' safety when his ex-wife Britney Spears went into a meltdown this year. The toxic singer was hospitalized for psychiatric assessment last January after she reportedly refused to hand over her children following a supervised visit. Spears was in the middle of a bitter custody battle with Federline at the time of the incident, and he confesses he was worried about his children when she hit her "lowest points."He tells Hello! magazine, "Yes, I was a bit concerned. In my heart, I never felt she'd ever do anything wrong to the children. But some of her lowest points were my lowest points, too. It has been tough for me to go through all this at the same time." Describing the moment he saw Spears being carried away on a stretcher, Federline adds: "That's one of my lowest points right there. It was stressful. But we're all through it now."And Federline is convinced his and Britney's problems were made even worse because their marriage was in the public eye. He says, "It's tough. It definitely doesn't make it any easier. But when we were together, we had each other to rely on. And we had lots of moments together that ... well, people don't know what goes on behind closed doors. We had a bit of private life."