"I taught them to walk up to any young guy who's shoving you around on the playground and scream in their face that it's not okay to be mean," the actor said Thursday while promoting his action flick, Red, in England. "[It] works, too"
But Willis, 55, who wed for a second time in April 2009, isn't afraid to play the tough guy when necessary.
He joked on the British TV show Daybreak, that he'd "kill" anyone who messed with his children, and said his kids "keep me young" and make "fun of my age." His girls – their mom is Demi Moore – also tease him about his baldness.
"They like to play with my hair," he said. "And from time to time I'm forced to remind them that maybe I just got lucky because I don't have to spend so much time getting my hair done."