I want to warn you before you start this craft: it gets a little messy! :)
We started by dumping a little cornstarch on the table in front of each of the kids. (About 3/4 of a cup)
Add about a cup of shaving cream to the top of the cornstarch. Then have the kids pick a color of food gel. Drizzle some of the gel over the shaving cream.
Now have the kids start working the ingredients together. We had to add more shaving cream to get the dough to form. As you work with the mixture, if it feels crumbly, add shaving cream. If it is very wet, add a little cornstarch. (A little goes a long way.)
When we finally got it to the perfect consistency, it felt exactly like the moon dough you buy in the store.
(Yes, this did get on the floor. It vacuumed up pretty easily though. However, you might not want to do this in a room that has carpet.)