After the clay has been kneaded, place vaseline inside a plastic food bag and spread until sides are covered. Place the clay inside and place in the fridge for an hour or so.
(If there is any left over after activity, keep in fridge in plastic bag taking out any air from the bag as it is air drying clay.)
Take the clay out of the fridge and create little balls, when you have enough balls, mix a little food colouring to each ball and knead the colouring into the clay, roll back into little balls. With a coctail stick pierce the balls all the way through. Leave to dry overnight.