Britney Spears' Ex Kevin Federline Says He's Rooting For Jamie Lynn On 'DWTS'

Britney Spears has a rollercoaster relationship with her sister, Jamie Lynn ... but Britney's ex-husband is fully on Team Jamie Lynn as she competes on "Dancing With the Stars."

Kevin Federline says his family -- including his two sons with Britney -- have been watching Jamie Lynn on the ABC dancing competition show and she's got their vote.

K-Fed, Jayden James and Sean Preston recently moved to Hawaii ... along with Kevin's wife, Victoria ... so the time difference makes it hard to watch Jamie Lynn live on 'DWTS' ... but he told a photog on Oahu the fam watched Jamie Lynn after the fact.

Britney and Jamie Lynn have had their share of beef over the years ... but as we first reported, Britney got a heads up her little sister was going to be on 'DWTS' and Britney was fully supportive and, apparently, even excited for JLS ... and now we know K-Fed and the boys feel the same way.

It's been about two months since Kevin moved the boys to Hawaii, where they're living in a new rental home, and K-Fed says he's got no complaints about their new surroundings.

Kevin and Victoria were on their way to The Lei Stand, a cocktail lounge in Honolulu, with another couple Monday night when he talked to a photog ... and he's already giving off island vibes.