First, set this craft up outside. Inside, using 5 gallon Spackle buckets to mix the concrete, is a bad idea. It's way too hard to keep spills from happening. At the Community House they set up four tables, each covered with butcher paper, in a shady spot under some trees. Not too hot. Perfect.
Second, mix the concrete in a wheelbarrow. A wheelbarrow with 2 wheels in front is preferable, being less tippy. Mix the stuff with a shovel. Set up the wheelbarrow near a hose.
Third, instead of expensive craft-store molds, buy cheap plastic plant saucers (those things that go under potted plants to keep the water from going all over the place). These are usually about fifty cents apiece. You don't need to grease them with Vaseline because you can flex and bend them to get the stepping stone out.
Fourth, BEFORE filling the mold with concrete, write the child's name on the plant saucer with a sharpie.
Fifth, have two adults hold the mold while it is being filled with a scoop of concrete since it is too heavy for a child. The plant saucers are not as sturdy as an official Stepping Stone Mold would be. Then have the two adults carry the mold over to one of the paper-covered tables in the shade so the child can decorate the stepping stone at his/her leisure.
Sixth, you can never have too many decorating options. They had piles of shells in all sizes and shapes, beach glass, little hearts and other shapes, glass pebbles, feathers, sprigs of pine... everything you can imagine. There were bags of all kinds of goodies scattered around on the tables and the children got to decorate to their hearts' content.
Seventh, and this probably goes without saying, clean up the wheelbarrow and shovel with water immediately after you are done. Don't let the stuff harden! On the other hand, set the stepping stones in the sun overnight (since you do want them to harden) and in the morning, twist them to pop them out of the mold.