This post is advocating make-up as a tool/instrument for artistic and self-expression given situational context.
I think it is important to set boundaries about what is appropriate and what is not. Answering the question: "what are you trying to say" with the way you are presenting yourself can help start conversations about what kind of make-up is appropriate for the situation.
Kids can learn about colour, highlighting, lowlighting, outlining to draw attention to or away from something of interest through the use of make-up. There is also the opportunity to "try-on" different personas. I remember being introduced to make-up as part of a school play and wishing I could look like that all the time. However, make-up appropriate for the chorus in Pirates of Penzance (tarts) is not appropriate for school, which I teach my children is the equivalent of the workplace.
They can also learn that it takes time. If you want to wear make-up to school, all the other preparations need to be done first. Repairing works of facial art can be aggravating. What happens after swimming lessons or sports? This might encourage them to be better time managers and also to prioritize function over form.
I encourage all ages of my Girl Guides to experiment with make-up because it helps them practice their hand-eye coordination and explore different aspects of their self-identity. They are also much more forgiving when they make mistakes on themselves than when I do (i.e., "A butterfly doesn't look like that!).
So, in short, it's worthwhile to ask "what is the purpose of the make-up?" and encourage context appropriate use. By encouraging appropriate use, it gives parents/care givers the opportunity to comment and give feedback on their choice rather than support secretive, uncensored behaviour.