If you want low-key and inexpensive, try St. Louis (yes, I said St. Louis). St. Louis has more free/low-price family attractions than any other city in the US except for DC. The zoo, the Mo. Botanical Garden, the Arch, the City Museum, The Magic House, the Science Center, Grant's Farm, Laumeier Sculpture Park, Forest Park, the Butterfly House and Carousel, and a lot more I can't think of at the moment.
For the grownups, there are also several pro sports teams, a number of fine museums, two beautiful cathedrals, an excellent cultural offerings, lots of great restaurants, several casinos, and of course, the A-B brewery tour.
You also should take them to Chicago; between the Children's Museum, Shedd, Lincoln Park Zoo, and Science and Industry, they will have a ball. Even the L will be fun for a kid who doesn't normally ride trains. I also find that little ones just enjoy the pulse of a busy city like Chicago; they get wide-eyed just watching all the people.