Mix up some coloured water in a couple of cups/glasses by adding a few drops of food dye to the water. Make as many colours as you like.
Pour coloured water into ice cube trays, freeze.
Once ice is frozen, warm up some milk and pour it into a container or tray. You don't need much, about a cm deep is fine.
Add ice cubes to the milk and use a fork/spoon to slowly move them around. As they melt in the warm milk they will leave pretty trails of bright colours! Your child can experiment with moving them faster and slower - slower gets the best result. See if you can make any cool patterns and shapes.