After the containers are decorated and personalized, the children chose their plant (one cell of a six-pack or 4 inch pansy, veggie, or other annual). Potting soil was put into a large, somewhat shallow plastic storage bin (like those used for under bed storage). This allows easy access for several children at one time and seems to reduce the amount of mess. Kids can put their pot right into the bin to fill, or next to it, for easy clean-up. At the Mom's Club meeting, we put the bin directly onto the floor for very young participants (18 month to 3 year olds) and that seemed to work great.
After they put some soil into the pot, an adult assisted putting the plant in and then the children finished adding more soil. At both events, we did not have the children water their plants to minimize the mess but advised them to water them when they got home.
Other things to consider during any children's gardening event is to have your staff explain what type of plant they are planting, the plant's name, and general care and watering instructions. Most kids (and some parents) don't realize that the plant might need direct sunlight and regular watering. Use this opportunity to teach while the kids are playing in the dirt and having fun.