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Kevin NEVER takes the kids to Britney's house but yesterday, he did just that!

It's usually Kev's bodyguard who drops off Sean and Jayden at their mom's for visitation but Thursday afternoon, KFed himself chauffeured the boys to Brit's Summit residence, which means BRIT AND KEVIN MUST HAVE BEEN FACE-TO-FACE ... FOR THE FIRST TIME IN A LONG WHILE!

Then Brit took the boys out during the day -- in her own black SUV, with her own bodyguards -- before sending the kids home to Kev.

What's changed between Britney and Kevin that would inspire him to visit his ex?

more pix inside ...