It might appear obvious, but you would be amazed at some of the errors made out there.
In May 2012, a DJ and pool attendant from Brazil named Paulo Henrique dos Santos decided to paint himself as the Incredible Hulk. His green skin looked amazing, right up until the moment when the party was over.
That's when he discovered in the shower that he'd used the wrong kind of paint. His body was covered in what was usually reserved for daubing ballistic missiles and nuclear submarines!
After taking about twenty baths and showers, scrubbing with all that he could lay his hands on, he did what anyone would do in this situation. He called his Mum.
After unsuccessfully attempting to remove the green paint herself, Senora dos Santos organized back up. For over twenty-four hours, a group of neighbors and friends took it in turns to scrub the hapless Hullk.
Finally the green paint came off.
On the assumption that you are not willing to put your own child (or yourself!) through this, then it's better to find the correct face paints in the first place. There are plenty on the market and the reputable ones meet American and European marketing standards.
They are designed to be applied and removed easily, without damaging your kid's delicate skin. Being water-based, they simply wash off under the tap.