This is a frequent question!
The best way is with warm water and a little gentle soap - I use 'dove' as it is gentle on my kids faces.
There may be a little residue/staining left with certain colours such as red, but this does come off quickly with a few washes!
For lingering colour, use oil free eye make-up remover. If you have already tried to remove the paint with another method and there is still lingering colour, apply a moisturizing lotion, wait at least 30 minutes, and wash again.
There are a wide variety of methods available to remove face paint with products found around the home or easily purchased from a local drug store. Generally, no matter which you chose to apply, you will need cotton balls for application as well as warm water and a washcloth for rinsing.
Another method for removing face paint is by dipping the cotton balls in baby oil and gently using the oil-soaked cotton to wipe away the paint. However, this may require the use of many cotton balls, depending on how much paint is present.
Baby shampoo is another good choice, as it is mild enough so as not to irritate the sensitive skin of your face. Work the shampoo into a rich lather onto the washcloth, and then gently remove away the paint. Rinse your face with warm water, and then dry. If you don't have baby shampoo on hand, use the mildest soap you have, preferably one with a neutral pH, and be careful to avoid your eyes.
TIP: Try not to sleep in your facepaint as it can rub off onto bedcovers.
Removing facepaint from clothing
To remove face paint from clothing, wash it by hand in cold water using soap or pre-washing agent. Do not try to remove the makeup from clothing in a washing machine. Avoid warm or hot water when trying to remove the paint from clothing. Avoid using near sensitive fabrics like silk that absorb pigments.