I took the time (2 hours) to read this entire thread. There are many good reasons to teach many fine concepts to children, but the one overwhelming obstacle is that you will have to convince Johnny that writing his how "game" is better than waiting for EA or UbiSoft to come out with yet another title that costs double the cost of a R-Pi.
Getting children engaged will be difficult is this enviroment where children are raised to expect magical entertainment from these devices with no input from themselves. I have seen suggestions from BASIC, through Python, C++, even SQL, COBOL and ADA (Seriously, do you think any kid is going to last 10 minutes in an Ada class? Or in a non-graphical environment for that matter?) Probably not. You also don"t want them in a class that will bore them the other way either. LOGO might be great for 4 year olds, but not many 4yo potential programmers are going to be connecting their first DIY R-Pi to their TV.
Think 10-14 year olds.
I was looking at the programming tools available in Android and Iphone / Ipad worlds also to get ideas (face it, where this is being marketed to is not an ADA, Algol, COBOL world) and there are some really good programming environments out there with libraries and SDKs to do just about anything one could want creatively.
One caught my attention. CODEA for Ipad http://twolivesleft.com/Codea/
is IDE/Interpreter and Compiler for kids just starting out with LUA. Check it out. In concept, it looks like something that would be ideal for R-Pi. It really wouldn"t matter if the underlying language were BASIC, LUA or Python, but I think it should be one of those three. the Codea project looks nice enough that I think it would suffice perfectly to clone that effort into the R-Pi.
Also, look at http://www.appgamekit.com
It seems to be a system similar to Codea, but centered on BASIC. One of the possible draws to children today are not trying to code the next Battlefield 3, but rather the next Angry Birds or "Chuck the Monkey" or anything. If I were 12 and thought I had a chance to make a SIMPLE game that I could sell for $1.00 that would sell 25 million copies... (or as many R-Pi's are sold ), I would have spend 18 hours a day coding.. Unfortunately, in my day, the best you could program when I was 12 was Lunar Lander, Zork or Star Trek on a TRS-80. Any of these new tools are more than capable of making the next Bird game.
I was also looking at FREEBasic.net and it has a really nice interpreter / compiler / IDE as well though the interface is not as captivating as Codea. I realize neither of these options are immediately available in their current form for the R-Pi, but from the discussions that some have given on this thread, it sounds well within their abilities to write a package like Codea.
Just my 2 cents.