Here's a possible color mixing alternative...
Although you've been searching for quality and consistency in paint to teach
color mixing, the plastic-type clays like Sculpey can also demonstrate color
mixing--just smoosh a little red and a little blue together between the fingers
and violet should result. It's not paint, so I guess there are pros and cons
for using it for color theory primarily used in 2-D fashion, but it is "hands on
color theory" if you're willing to do a 3-D color wheel--Just an idea, although
I would love good quality and consistency in paints, too!