Helping celebrate their sons' latest milestone, Britney Spears and Kevin Federline put on a united front at their boys' preschool graduation.

The pop star, who was joined by parents Jamie and Lynne Spears, and her ex-husband arrived separately at a San Fernando Valley school Thursday, but met up briefly. Preston, 4, and Jayden, 3, were later treated to McDonald's Happy Meals after the ceremony, says a source.

Spears, 28, "was very excited about her boys' graduation and she dressed them up for the festive occasion," the source says.