First, please, please, please use only FDA approved, cosmetic grade FACE paints. Snazaroo, Kryolan and Wolfe are good brands. Craft paints may be non-toxic but they are not made for use on the skin and could be harmful. Cosmetic glitter is made of acrylic without any sharp edges, craft glitter is sharp edged and scratchy. If a child rubs craft glitter into her or his eye it will ruin your party.
You’ll also need high density sponges and good quality paintbrushes. It’s much more difficult to paint with cheap brushes, although they don’t have to be sable, synthetic brushes are fine. Start with a filbert, a flat, and a couple of rounds.
Practice some designs on paper, on your arm and then on a wiggly child. Older children can help but I wouldn’t let any little ones paint themselves.