Gwen Stefani and Gavin Rossdale kicked off their Saturday at a birthday party in LA with their kids, Kingston and Zuma. The foursome headed to a park for the bash, which included balloon animals and games. While Kingston and Zuma were playing, Gwen and Gavin snuck away and shared a sweet hand-holding moment. After they wrapped up the family fun, Gwen and Gavin got to work. Gavin and his band, Bush, appeared at the first night of KROQ's Almost Acoustic Christmas concert as did The Lumineers, Linkin Park, and others. While Gavin was performing "Glycerine," Gwen surprised the audience by joining him on stage to sing the hit. The duo closed out their show with a kiss in front of the screaming crowd.