Britney Spears saw her children during a weekend visit to Promises, PEOPLE has learned.

Estranged husband Kevin Federline brought their boys, Sean Preston, 17 months, and Jayden, 5 months, to the Malibu rehab center Saturday evening, a source says.

"The kids are his No. 1 priority and they always have been," the source says. "He's so stressed out right now. He only wants the best for Britney and the kids."

This was the second time Federline has visited his estranged wife. He also went to Promises on Friday without the children.

Spears 25, voluntarily checked herself into the residential rehab center on Feb. 20, left the next morning, then returned Thursday night.

The singer's father, Jamie Spears, 54, told FOX News from his home in Louisiana that his daughter is "a sick little girl. ... We're just trying to take care of her."