Visitors to Madonna 's $6.5 million Beverly Hills mansion on a recent afternoon could stroll through and behold master-works by Picasso, Diego Rivera, Frida Kahlo and...resident artist Lourdes Leon, age 6. Alas, Lourdes belongs to the enfant terrible school of art, which has her famous mom clenching her teeth in frustration. "My daughter spilled black paint on the bedroom rug," says a stressed Madonna . "We're not really sure how it happened. She was left alone for two minutes, and, well, it's not pretty." So where is her husband, British film director Guy Ritchie? "He's upstairs, working on a script," she says. "I'm much more the disciplinarian of the house, although Guy is getting more involved." Then, flashing a playful grin, she adds, "I'd have to fire him as a dad if he didn't get more involved. I can't be the hard one all the time." Madonna , looking to soften her image? Chalk it up to mo...