Britney Spears'estranged husband Kevin Federline took their two young sons to Las Vegas over the weekend - 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, 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."