Try Engineering is a site that hosts lesson plans and games designed to get students interested in engineering. The lesson plans are arranged according age and engineering topic. The lesson plans can be downloaded as PDFs. Be sure to look at all of the lesson plans because several are specifically for upper grades. This is just one of many sites I came across when searching for engineering sites for elementary students.
The games section of Try Engineering features 36 online games. Some of the games were developed specifically for Try Engineering while others are hosted on other educational sites like those of NASA and PBS.
The games section of Try Engineering also includes links to a dozen iPad apps that students can use to learn about engineering and programming.