The Active Schools coordinators are targeting the level 2 training (on-road, about P7) because it prepares pupils for the move to secondary school and probably independent travel to school.
They don't have the trainer resources in Edinburgh to be able to offer all three levels, hence the prioritisation. Indeed they offered free training to volunteers in the south of the city earlier this year (on to which I managed to sneak!).
As I recall, Level 1 (~P5) covers basic cycling skills, Level 2 adds traffic/road awareness and Level 3 (~S1) heads into journey planning territory. Level 3 tends to be provided in Bike Clubs, rather than being a formal training programme.
As for accessibility, our school cluster has bought decent bicycles for loan as part of the training, and our Active Schools coordinator is looking into buying balance bikes for use with younger children as well.
If I spot the council offering training to volunteers again, I'll post it to here.
Robert