The Musée d'Aart Cmerounais in Yaoundé has a very good collection of traditional masks apart from items showing bronze and wood work. Some may appear scary but you will enjoy going through the collection.
The Museum of Douala is place where you can with your kids. It has natural history and prehistoric galleries, which allows you to get an idea about the historical background of this place. Another place where you can visit with your family is Musa Heritage Gallery.