Colour mixing with ice cubes

My son has recently started doing a few hours a week at preschool and he's usually exhausted afterwards, with several hours to go until his bedtime.  I've been trying to find engaging, but not too challenging or physically tiring, activities to give him something to do which doesn't involve watching TV for ages.  One such activity which worked nicely was some colour mixing with ice cubes.

I froze fish shaped ice cubes with water plus food colouring in each of the primary colours (red, yellow and blue).  I found six identical small glasses, put some white paper underneath, and helped him put fish into them as follows:
1. blue
2. blue and yellow
3. yellow
4. yellow and red
5. red
6. red and blue

Ready to add water to the coloured ice

He was so tired that he didn't ask why he was putting frozen coloured fish shapes into glasses, he just followed the instructions I gave him!  When he'd finished allocating the ice cubes, I asked him what he thought would happen to them. He thought (correctly) that the fish would melt, and then it dawned on him that something interesting might happen to the colours so he asked for water and a pipette.  He then laboriously transferred water from a beaker to each glass in turn with a pipette... once a bit of melting had happened in the cold tap water, he saw the colours mix in the glasses where there were two coloured ice cubes and excitedly told me what was happening.  He also realised that he could speed the process up by adding water more quickly and decided to pour instead.  After a couple of beaker refills (and a bit of water on the table) he had six different coloured glasses of liquid - blue, green, yellow, orange, red and purple.  He was very proud of his rainbow of coloured solutions and insisted that we kept them to show his Dad later.

Pouring, not entirely accurately

There's some nice simple science learning in this activity with melting, rates of melting and primary colour mixing. I wasn't sure when we started the activity whether he'd be interested given how tired he was, so I particularly enjoyed watching him work out what would happen and figure out a way to speed it up.