(licensed under the MIT License with parts under the Apache License)Etoys is...
*an educational tool for teaching children powerful ideas in compelling ways
*a media-rich authoring environment and visual programming system
*a free software program that works on almost all personal computers
There is a portable version, Etoys To Go, to be run from a flash drive. It does not install the browser plugin and creates no file associations. I have not tested whether it is stealth.
There are loads of activities and games created with and for Etoys in the showcase section of the website.
Maybe one or the other teacher is around here and interested in that.
Download: http://squeakland.org/download/