The project, scripted by David Diamond and David Weissman, reunites Travolta with his "Wild Hogs" director Walt Becker.
Travolta and Williams play best friends and business partners who somehow find themselves stuck taking care of seven-year-old twins.
Of Mac's character, The Hollywood Reporter says he's "Johnny Lunchbox, the Elvis, Bono, and Beatles of children's entertainment all wrapped up into one super-charismatic puppeteer."
Mac was most recently seen in small roles in the summer hits "Transformers" and "Ocean's Thirteen" and in a bigger role in the spring swimming epic "Pride." The long-time star and producer of an eponymous FOX comedy, Mac also signed on this spring to produce the reality pilot "Welcome to the Family" for NBC.