To make clean up easier, put a small plastic sandwich bag inside the paint container. Make sure the top of the bag hangs over the edge of the container. Snap the lid on to secure the plastic bag. When directing behavior, always keep the expectations successful for each child. Some kids can wait successfully for 5 minutes while others can wait only one minute.
Positive reinforcement is the key to behavior guidance as well as prevention.
Think about what you could change in your environment, routine or even yourself before thinking about changing a child. Rest time means different things for different children. We need to respect that. Have fun yourself. If they see you having true fun they will too.