I've worked out via chart exactly how old my sims are during each point of their lifestage so that I can pay attention to things like this. I don't put make up on child sims under 10, however, I myself wore eyeliner and lipgloss from age 11, so I don't see anything wrong with my more 'mature' sim children putting on the same around that age. I dress my child sims differently so that they appear to 'grow up' during their childhood like people do (you go from 4-12, there's a pretty big jump there! lol). It also helps me identify at a glance (more or less) roughly how old a child sim is. 4 year old sims wear more babyish clothes, onesies and the like, and pastel colours, dinosaurs, that sort of thing, while 10-12 year olds wear skinny jeans, hoodies, mini skirts etc. that are more stylish and more teen-aged, so make up goes along with that quite well for certain sim kids.

I don't tend to use makeup on kids, besides eyelashes when they transition to toddlers, though I have used freckles or similar before. Sometimes I even forget to add more makeup to my teen sims!