Add cornstarch and icing sugar to a medium bowl and stir them with a spoon.
Spoon some marshmallow fluff into the bowl containing the dry ingredients. If you'd like the slime to be coloured, add a few drops of food colouring.
Gently stir the ingredients together with a spoon. Once you've gotten as far as you can get with the spoon, use your hands to continue mixing the ingredients. You can add a bit of coconut oil to your fingers before you start to help the slime stick less.
Pour coconut oil onto the slime and work it in with your fingers. Then, give it to your child to play with!