A lovely little experiment today that is nice to watch for a while... You will need: food colouring (or some kind of dye) in red, blue and yellow (the primary colours), 6 small cups/containers (they don't need to be the same shape, but don't make them too big because it will take too long!), some water, 6 kitchen towels and a jug for pouring.
Collect your 6 cups and place them in a circle. In the first cup put some red food colouring in, the second cup put some yellow and the fifth cup pop in some blue. Now pour water in each cup so they are roughly 2/3 full. Leave the cups in between empty.
Fold your paper kitchen towels into long thing strips. Now place each end into the cups (so one end goes into the colour cups and the other into the empty cup next to it (see photo below)
You can watch as the colour starts to move along the paper straight away and within around 1 minute, all the pieces of paper turn yellow, blue or red. This is a good chance to ask your child what will happen next. A few minutes later, you will see the water starting to drip into the empty cups and mixing to make orange, green or purple. After around 5 minutes, you should see the water level in the cups go down and the empty cups should have some coloured water in them. Leave it for a little while and the new mixed colours will start to move along the paper towels too. In 20 minutes all the water levels should be roughly equal and the colour mixing should be complete.
A bit of trouble shooting: if this is happening slowly you might want to double up on the paper towels so they draw more water (some brands work faster than others). You do need the glasses to be pretty full so another thing you could do is add more water to the primary colour cups.