It appears that things have gotten so serious between Blake Shelton and Gwen Stefani that her kids with Gavin Rossdale have reportedly started seeing him as a second father figure.

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According to Hollywood Life, Stefani's kids, Kingston, 9, Zuma, 7, and Apollo, 2, are even calling Shelton dad—which isn't sitting well with their actual father.

"The moment Blake walks through the door, those kids are climbing all over him like it's a WWE tournament," sources said. "He's so incredible with her kids, taking them on dune-buggy rides and playing games like laser tag. You'd think he'd a raised a football team of rowdy boys!"

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"Everyone knows Blake is a big kid, so it's no surprise that they adore him," the source continued. "And he loves those boys just as much as he loves their mother."

But, or course, that is not sitting well with Rossdale, who is reportedly not happy his kids have bonded with the country superstar.

"Blake is amazing with the kids. He's such a big kid himself and loves doing all the same things as they so," a source said. "One of his favorite things to do with them is go on hikes—they love Malibu Creek. They get out there in the mud, hunting for frogs or playing hide-and-seek. Blake loves any excuse to get outside and get dirty, so he's a big hit with Gwen's kids. It drives Gavin nuts that his kids have bonded with Blake. He hated that they're spending so much time with him. He doesn't have any say in it...and it's killing him."

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Blake Shelton, Gwen Stefani