If you do not want the colours to mix, be sure to wait for the first layer to dry before applying another colour over it or up against it.

Hint: Regularly change the water you rinse your brushes in and preferably work with two water bowls: one for light and one for dark shades.

To create a wide mouth, draw it in using a dark colour right over the cheeks. You place a light colour up against it and then put a darker stripe around that, which highlights the lighter part further. By giving the cheeks a light colour, you create an extra effect of roundness and width.
Make the nostrils larger with the dark colour.
Grimas Water Make-Up (Pure) can be used around the eyes, but children often do not like it when brushes are so close to the eyes. So decide yourself how far you want to extend the colour.
When working under the eyes, have your model look upwards - only with the eyes, without moving the head.
If you work above the eyes (eyelid), have your model close his/her eyes in a relaxed manner. Children are inclined to shut their eyes tight. Then you can say: keep your eyes closed as if you are sleeping.
You add contrast to the patches by accentuating the outer edge using a dark line.