At Madonna‘s L.A. concert Oct. 10, Heidi Klum was accompanied by two bodyguards-one on duty, the other her date. As Klum and her bodyguard-turned-boyfriend Martin Kirsten made their way into the Staples Center, “they had a driver-bodyguard so they could enjoy the night together,” says one onlooker. “They shared cocktails and were dancing for most of the concert. She and Martin were kissing and looked very happy together.”

Just three days later the couple stepped out again under very different circumstances. Surrounded by Klum’s four children-Leni, 8, Henry, 7, Johan, 5, and Lou, 3-and her parents, Gunther and Erna, the pair attended Henry’s soccer game with “Martin keeping an eye on all the kids and making sure that everyone was safe,” says an observer. “He kept things very professional.”

Since the Project Runway host, 39, went public with her new romance-telling Katie Couric on her talk show, “He’s a great man, and recently we just got to know each other from a completely different side”-the South African bodyguard, 40, has been juggling a dual role in Klum’s life. He’s still a full-time bodyguard who reportedly remains on Klum’s payroll. But the role Kirsten has clearly occupied since Klum and Seal announced the end of their seven-year marriage in January? “He’s a huge fixture in Heidi’s life,” says an insider. After Klum filed for divorce, “He was a huge support to her. He helped her through a very difficult time in her life and empowered her to move on to what’s next.”

A romance wasn’t in the original plan. When rumors first surfaced this past summer that Klum and Kirsten’s relationship was moving from professional to personal, a friend close to Klum told PEOPLE at the time, “It’s complicated.” While Klum isn’t the first star to fall for her protector (see box), the friend insisted, “Their relationship is hard to define,” adding, “She’s not ready to announce to the world she has a new boyfriend.” But as time went on, “Heidi enjoys that Martin brings stability to the family,” says the family source. “Heidi was sick of the constant drama with Seal. She is happy to be with Martin and feels very lucky that he gets along so well with her kids.”

And she’s not afraid to let the world see that. There was Kirsten backstage at the Justin Bieber concert Oct. 3, gently carrying a sleeping Johan. There he was again at a West Hollywood pumpkin patch on Oct. 6, watching Klum spirit Lou down the superslide. Then on Oct. 9 the entire family, including Klum’s parents-who “get along great with Martin” says the family source-celebrated Lou’s third birthday at Disneyland. “Heidi and Martin have occasional solo dates, but their life together is mostly about Heidi’s kids,” says the family source. “He went from being a single guy to acting like a stepdad to four kids, and he is not freaking out about it at all.”

As for Seal-who considered Kirsten “a dear friend” and was “upset” when he first heard about the budding relationship, according to a source-he is still far from happy about Heidi dating Martin, says the family source. “But he does appreciate the fact that he at least knows who the kids are spending time with.” Kirsten has proved that Seal has nothing to worry about. “Martin can be very playful but can also set rules for the kids,” says the family source. “His best quality is that he’s always up for anything. He likes experiencing new things with Heidi and the kids.” Just don’t think the couple are jumping into anything too serious too soon. Says the Klum friend: “They’re taking it one day at a time.”