I think your idea of creating an activity of teaching the child about handwashing is a creative idea. For little kids, for them to learn something it is always useful when you make it as a game. Using the black light to show them the germ that are still on his/her hand will amazed them.
One way of spreading MRSA is by skin-to-skin contact because it is a skin infection, which is why it is so important to teach the child the important of handwashing. However, if a child is in a pool he/she will not be infected with MRSA. MRSA does not survive long in pools or other swimming places, such like hot tubs/spas because of disinfectant (chlorine) and the pH level that contains in the water. Although the child may not get infected of MRSA