Growing up fast: Brad Pitt plays the doting father as he takes blonde twins Knox and Vivienne to the park

By DAILY MAIL REPORTER Doting dad: Brad Pitt takes his twins Knox, left, and Vivienne to the park in Hollywood It costs Brad Pitt and Angelina Jolie a whopping $10million a year to raise their six children.But despite the huge bills for nannies, language tutors and private jets, the youngsters prefer nothing more than spending time with their parents.Brad treated his youngest children, twins Knox