A Brief History of Utah
This is a great history of Utah in essay form made just for kids. It begins with the state's prehistory and goes all the way to the present. It also shows pictures, gives access to unfamiliar terms, and has suggested reading to get further information.
Source: Utah State Historical Society

Historical Timeline
This site has the all the same great facts as the above essay, but it is set up on a timeline.
Source: Utah State Historical Society

Utah History Encyclopedia
This is the first complete history of Utah in encyclopedia form. It consists of 575 articles and over 200 historic photographs.
Source: University of Utah