That's what I'm asked a lot of times.
In most cases I use chalk pastels. It holds quite well, is easy to apply (I use a brush, as it allows you to apply more precisely and with smoother gradations) and you can see the result directly and immediately. It is probably not washable (I haven't washed a figure yet), but that's usually just a little problem, you can just dye the cheeks again after washing.
To my knowledge chalk pastels are also not harmful to health, but this can vary from manufacturer to manufacturer. But since many artists work with it and also inhale dust from the chalk when painting, this would surely be known, especially since we are talking about tiny quantities here.
The nose on the right was painted with this chalk :o)
If it should be more durable, i.e. not suckable (I haven't tried washing it too), then I use acrylic paint markers. These can be thinned with water to the desired shade, of course also mixable, cover - if it should be stronger - very well and can also be applied very well with a brush. The downside is that you need a little practice and the actual color tone is slightly darker at first, since the color is wet. You can't correct it if it's too dark, too light is no problem, just apply more. With a little practice, that's not really a problem.
Unfortunately, my red pencil is currently missing, actually I still have a real red one. Burgundy red or hazel as shown in the picture gives also beautiful cheeks.