Cream 2 cups of icing sugar and ½ cup of butter with a mixer.
Add the milk and continue to mix.
Add the remaining 2 cups of icing sugar and the vanilla.
Adjust to desired consistency with extra icing sugar or milk if necessary. You want it thinner so your kids can spread it on the cookie!
You can add food colouring to make it all one colour or split it up into multiple bowls and make multiple colours.