Britney Spears' estranged husband KEVIN FEDERLINE took their two young sons to Las Vegas over the weekend (ends04MAR07) - for a promotional appearance at a nightclub. Federline is watching the couple's children Sean Preston, 18 months, and JAYDEN JAMES, six months, while Spears is seeking treatment in rehab. Federline was in Las Vegas for a long-planned paid appearance at the Mirage Hotel + Casino lounge Revolution. A source tells American publication People, "Kevin was really low-key. You could definitely tell he was on his best behaviour. "He was having a good time, but wasn't the wild, crazy Kevin who used to come to Vegas." The former back-up dancer arrived on Saturday (03MAR07), along with an unusual entourage, featuring a nanny, his mother and brother and two bodyguards. Although he was in town for a party appearance, Federline appeared to have his priorities straight. The source adds, "He was late for dinner (with friends and family) because he wanted to tuck his kids into bed."