Add a few drops of food colouring and mix some more, until the colour is visible throughout.
Cover the bowl or container and place it in the fridge. Let the chia seeds soak for 3-4 hours, until they develop a thick, slimy texture. Then, give the mixture to your child to play with!
Notes
White chia seeds can be replaced with black chia seeds, but the colours won't be as vibrant.