01/18/2009 AT 9:15 PM ET
The bash, whose theme was inspired by the classic children’s book Where the Wild Things Are, was held at the singer’s Beverly Hills home. Pal Nicole Richie, boyfriend Joel Madden and daughter Harlow, whose big day was on Jan. 11, were among the 40 guests at the casual backyard celebration.
With surrounding trees decorated in festive blue, green and pink balloons, party guests nibbled on kid-friendly food, including mini peanut butter-and-jelly sandwiches, catered by Los Angeles-based caterers Along Came Mary. And as a special treat, a source says the party was attended by costumed mythical characters from the book.