Gwen Stefani's eldest young son helped facilitate the No Doubt frontwoman's surprise appearance with Pharrell Williams during his set at the Coachella music festival earlier this year (14).

The music superstars marked the 10-year anniversary of their hit collaboration Hollaback Girl in 2014, and Williams invited Stefani to sing the track with him during his gig at the popular California festival in April (14).

During an interview with comedienne Ellen DeGeneres on her U.S. talk show on Monday (24Nov14), Stefani credited her eight-year-old son Kingston with bringing the pair back together.

She says, "It's just so crazy because he (Williams) keeps bouncing in and out of my life in such an amazing way... We hadn't seen each other in a while and all of a sudden he's doing the song Happy.

"My son (Kingston) would come home and be like, 'I wanna watch that song', and I'm like, 'Well, I know him. He's a friend of mine.'"

Stefani admits Kingston was "so mad" that he wasn't in the music video for Happy, so she suggested they make their own video, shortly after she gave birth to third son Apollo earlier this year(14), and they sent it to the superstar producer/singer.

She continues, "I sent it to Pharrell and he wrote me back and said, 'I know it's too soon, but come out and play with me at Coachella.' So that was the first time I had left the house - was walking outside and playing with him. It was amazing."

Their onstage reunion inspired Williams to collaborate with his old friend once again, and they recently released a new track called Spark the Fire, which has become the theme song for the new Paddington film.