This IS a fantastic museum, as all the other reviewers said, not just for those of us that grew up in the city and were lucky to have it close enough to visit frequently, and it's equally fascinating for adults, as it is for kids.
But, if you're with a small kid who isn't yet in control of their bladder functions, you might want to bring a diaper for them - I just went this past weekend with a friend and her 3-year old daughter. Thank goodness the 3-year old had a strong bladder - we were in the gems room on the Lower Level when she decided to tell her mother she had to go and couldn't hold it any longer. There are NO bathrooms on the lower level. The closest one required us to wait for an elevator, go up a floor, and then walk and walk and walk. The maps they hand out are very poor for navigating, and every guard we asked gave us such complicated directions we had to keep asking the next guard, and somehow wound up passing the CPW entrance and going to bathroom which was basically walking around the entire museum.
Apart from being very difficult to navigate, they do have fascinating exhibits, fun films, and enough varied animal, human, and dinosaur information to satisfy anyone.