When visiting Yellowstone National Park, Old Faithful is (of course) the must see attraction. But make sure you don't miss Mammoth Hot Springs. Tower Spring should also rank high on your list of things to see.
Don't miss the geysers, and take some time to explore beyond Old Faithful when you do. It's a weird place, and very interesting. Other famous sights are the Grand Canyon of the Yellowstone and Mammoth Hot Springs. Just driving around the park is an attraction in itself, anytime you might come across a heard of buffalo.
There are lots of options in Yellowstone, depending on what interests your family, the age of the kids, etc. There are a number of terrific websites that detail attractions and features of Yellowstone and the surrounding area that you will find helpful in planning our trip.
Among the best places to see in Yellowstone Park, the Old Faithful Geyser and Upper Geyser Basin are at the top of the list. Don't dismiss the nearly-as-popular Lower Geyser Basin, though. The Grand Canyon of Yellowstone and Yellowstone Lake are also among the best sites in Yellowstone. Hayden Valley and Mammoth Hot Springs are also popular places to visit.