09/29/2008 at 01:00 AM EDT
' sons Preston, 3, and Jayden, 2, may not be old enough to always remember their monster-truck-themed joint birthday party held on Sept. 13, but the bash capped off a week their mom won't forget: Three days earlier, Britney's sister Jamie Lynn Spears
arrived in Los Angeles to introduce baby daughter Maddie Briann to her cousins. "Britney's wanted this for a very long time," says a source close to the family.
It was the first time since Maddie's June 19 birth that the entire Spears family—parents Jamie, 56, and Lynne, 53, brother Bryan, 31, Britney, 26, and Jamie Lynn, 17—has been together. At the party Britney doted on her boys, who cruised around in motorized trucks and jumped in a bouncy house. "You'd never know it was the same girl who shaved her head and had her kids taken away," says a partygoer. (Britany's ex, Kevin Federline
, 30, didn't attend.) "She just looked fit and trim and cheerful." The good mood extended to the rest of the clan, who can finally relax after more than a year of drama. Says the partygoer: "They were all hanging out, just having a ball."