The Jonas Brothers' reunion was almost spoiled -- by someone in their own family! Luckily, she's too cute to stay mad at for long.

The brothers were guests on The Tonight Show With Jimmy Fallon on Wednesday, where they dished on their new album, Happiness Begins, and getting back together as a group. According to Nick, the secret reunion was in the works and kept under wraps for almost a year -- but was almost spoiled by Kevin's adorable 5-year-old daughter, Alena.

"She did tell her entire school that the Jonas Brothers were coming back -- before the Jonas Brothers were [officially coming back]," Kevin revealed.

.@KevinJonas’ 5-yr old daughter spilled the beans on the band’s reunion secret! More with @jonasbrothers tonight! #FallonTonightpic.twitter.com/cy6f4ASn6e

"We were trying to keep it a secret for basically a year," Nick explained. "We were making this documentary [Chasing Happiness], making the album. And we're like, 'We got this on lock. No one's gonna tell.' Kevin comes home and says, 'Guys, Alena told her whole class.'"

"Luckily, those 5-year-olds [were] tight-lipped," he added.

"They kept the secret for us," Kevin confirmed.

Family life has been a big part of the brothers' new musical venture -- since their last album release in 2009, all three JoBros have tied the knot and Happiness Begins is filled with sweet tributes to their wives and the joy they've found in married life. And Kevin's daughters, Alena and 2-year-old Valentina, clearly love seeing their dad as a rock star for the first time. The eldest Jonas recently shared a sweet clip of his girls dancing to his guitar playing and a photo of them rocking out in the crowd at the Spotify carnival celebrating the album's launch in Brooklyn last week.

"#happinessbegins seeing these girls made my night!" Kevin captioned the posts. "Love this so much."

See more on the Jonas Brothers' reunion in the video below!

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