It is quite easy to set up a mouth painting activity. Essentially kids will just use their mouths rather than their hands when holding the paintbrush.
However, you should be careful about kids swallowing paint, and the paintbrushes going too far back into their mouths. Perhaps it is better to do this exercise with older children.
For this exercise each student can bring in 2 or 3 corks for the end of the paint brush to be pushed into. This gives the student something larger to grip between their teeth and reduces the risks.