I'd like to add my $0.02.
I like block-orientated visual programming languages to help get younger children, ie. 5 or 6, started. I feel they minimise the obstacle of learning syntax, and don't have the problems of flow-chart systems.
Scratch is probably the best known block-language and is built on top of Squeak, which is a portable Open Source SmallTalk VM written in C by some of the original SmallTalk team. http://squeak.org/Download/
. I have not looked into it, and it will likely be some work, but poritng Squeak, and then Scratch may be within the resources of a few people for a few months to make it work. (It might work 'out of the packet' on RaspberryPi I really do not know). I wouldn't recommend spending much effort as Flash is coming (see below)
The Open University's new TU100 programming course will use Sense, an extension of Scratch (I was part of the team who designed the SenseBoard). Sense is designed to support undergraduate teaching, and so has some extra concepts beyond Scratch.
The other Scratch related block-language is Snap, AKA BYOB, http://byob.berkeley.edu/
which is also fork of Scratch. Snap let's you create new blocks, which is an important idea not available in Scratch. Snap is also used for undergraduate teaching. So I don't think children need feel they are using a 'toy' language.
. IMHO it is worth a read if you are a CS person.