If you take others with you on a business trip, you can deduct business expenses for the trip, but no more than if you were traveling alone. If, for example, your family rides in the back seat of the car and stays in one standard motel room, then you can fully deduct your automobile and hotel expenses. But you can't claim a deduction for your family's meals or jaunts to Disneyland or Universal Studios.
If you extend your stay and partake in some of the fun after the business is over, the expenses attributed to the nonbusiness days aren't deductible, unless you extended your stay to get discounted airfare (the "Saturday overnight" requirement). In this case, your hotel room and meals would be fully deductible.
Also, you can fully deduct the cost of your airline ticket even if it features a two-for-one or companion discount. For more information, see Deduct It! Lower Your Small Business Taxes, by Stephen Fishman (Nolo).