There is a great step by step lesson on http://www.askaboutireland.ie/learning-zone/primary-students/3rd-+-4th-class/science/bones-bodies-and-movement/all-about-you/whats-on-inside/
Just make sure to enable flash as there are great interactive activites.
PDST have good lesson suggestions here also:
BBC Bitesize have great videos which coincide with the above lessons at this link:
Game online for the children to complete in which they learn to label the human body and the functions of the body etc.
Video channel on youtube that has a funny song to explain all the major organs in the body