This is all you need. I've chosen a clear glass for demonstration.
Place one small drop of food coloring into glass. Depending on your child's age, you may have to use a glass or mug that disguises your PRE-preparation, OR simply hold it so you hand blocks the base.
To hide this drop you may have keep glass up on the counter. You want to do this step before they come in the room.
As you pour the milk into the glass, Leprechauns MAGICALLY turn it green!!!!
Magic green milk!