Sometimes you have to give it to the communists in my home country. One big brother said in 1984 computer needs to meet the kids and the official tv station started broadcasting BASIC language programming every night and schools started getting Apple computers and teaching BASIC. I was being more than lucky to have lived on a university campus and went to their grade school. After BASIC I also tried FORTRAN but didn't go all the way besides not having a compiler. I did remember learning c was hard in 10-11 grade but in twelve and college I became logical enough to do any c stuff and assembly. So for.you teaching c to 12 year old will be a bit more difficult than BASIC. On the other hand arduino will help keeping the kids interested. You could even spell out a bunch of digital writes on leds to make cylon eyes/knight rider. Later after they learned loops, you rewrite it with for loop.
Oh I have not used tube you mentioned except for crt tv 
I do teach electronics with tubes as they are much easier examples of current controlled devices than their semiconductor counterparts. How are you gonna miss one-way conducting?