Here are some websites on HOW TO GET IDEAS for your project:
http://www.ehow.com/list_6459433_invention-convention-ideas-kids.html
http://www.ehow.com/list_6556559_elementary-invention-ideas.html
http://www.inventions-handbook.com http://www.inventivekids.com
These websites show EXAMPLES of inventions by kids:
http://www.eduplace.com/science/invention/resources/real_inventions.html
http://www.slideshare.net/cmillica/examples-of-invention-convention
http://www.cnbc.com/id/42497934/Inventions_By_Kids
http://www.businessinsider.com/inventions-by-kids-2012-6?op=1