Could I put lidocaine cream to numb my son's face to take the non-washable paint he put in his face?

Not a good idea. While Lidocaine cream might temporarily numb his skin you will still have to scrub very hard to remove this paint. In addition to damaging his skin with all this rubbing it will start to hurt him once the cream wears off. As long as his skin is not red and irritated it would be better to gently clean the area with mild soap and let the paint come off slowly over a couple of days.