Cut glycerin bars into three pieces in advance. Each child needs one piece. If you're using purchased glycerin blocks, cut off two or three pre-measured chunks per child. Each child should wear an art smock.
Put the glycerin soap in the measuring cup and microwave for 20 seconds or whatever the package directions instruct. Continue to microwave the glycerin soap in intervals of 10 seconds until it melts.
Add several drops of soap dye for a light, pastel color. Add more drops if you want the color darker or bolder. Mix colors of dye to make a custom color. Stir the dye into the melted glycerin soap with a craft stick.
Pour the liquid glycerin soap slowly into the soap molds. If the child wants layers of different colors, pour one color layer. Let it cool for about five minutes and form a skin. Repeat this procedure for the remaining layers. To get a tie-dye effect, pour the uncolored liquid soap in first, then add a few drops of the desired colors and swirl slightly with a toothpick. The soap will harden in about an hour.
Release the soap from the molds after they have completely cooled. If the soap won't release, place it in the freezer for 10 minutes and try again.