Everything you need to understand or teach The War With Grandpa by Robert Kimmel Smith.
Peter Stokes learns that his grandfather will be moving from Florida to live with them permanently, and will be given Peter's room. Peter is outraged at this injustice and declares war on his grandfather. To his surprise his grandfather declares war, too. In the end, important bonds are forged and Pete learns about war and, more importantly, the importance of making peace.