RGB LEDs are great. They can produce a range of colours depending on the amount of power give to each LED bulb. Once you have access to the colour codes, you can produce 256 x 256 x 256 colours. Which is a lot of combinations.
We use Scratch For Arduino. This allows you to easily edit the analog power outputs. Put in a delay and a repeat and you’ve got a disco. As it’s Scratch based, it’s easy to do for primary children.