Not very simple, especially for kids, and yet intriguing and challenging: sounds are represented by geometrical shapes, and we are asked to discover the logic behind it (scroll down a bit in the page to reach the test). You can listen several times before answering.
After the test (or before it, or instead of it, or without any connection to it) arouse your kids' imagination by asking them questions about the connections between different senses: what colors can you "see" in different musical pieces? What sounds can we "hear" in pictures or books, or what kind of music might suit them? What shape does an itch, a scratch, a caress have?