Engineering Interact is a great site with interactive lessons for 9-11 year olds that help them explore and learn science and engineering concepts.
The activities fall under the following topics: Light, Sound, Forces and Motion, Earth and Beyond, and Electricity.
The activities are in game form and are well done. I played with two of them and had fun myself.
There is also information about how engineers use the science of that particular topic which is a great way to show them why the topic is important.
I was very impressed with these games and I can actually see them being used for older students up to freshman in high school as a fun way to learn or review the topics.
It was created and is supported by the University of Cambridge Department of Engineering.
The games and activities require FLASH, so you won't be able to use these on an iPad or iPod Touch.