I have been given a diminishing milk glass without any routine. Please can anyone help me with a routine suitable for children? Also can it be used with liquid other than milk as the cleaning looks like a lot of extra work and maybe damages the glass over time?

Bake a cup cake in a cereal bowl, or turn chocolate drink into wrapped candy you can pass out (check with moms) PM for method if necessary.

soy milk won't stain, and there are artificial substitudes available but not drinkable. Avoid anything with sugar like lemonade or soda. Don't use packaged cocoa mix.

if you want a dry demonstration of reducing calories from sugar (one I have done) -- use salt instead, or sand swiped from a gold course (ground marble)