I wanted to share this excellent website that will fascinate both you and your children. It's called Tone Matrix and it's incredibly simple to use. My 3 year old LOVES it, and I've heard feedback from people even saying their 2 year olds can't get enough of it.
It's a simple 16x16 grid that you can click each square to light it up. Once the square is lit up, it will produce that tone, ranging from low (at the bottom of the screen) to high (at the top). Left to right determines at which point in the "song" this tone will play. It's so confusing to explain, so really - just hop on over and try it out! It takes any jumbled up piece of nothing and makes it into something you'd hear from a symphony. The link to this webpage is: http://lab.andre-michelle.com/tonematrix
Let me know how it goes with your little musicians!
By the way, at the beginning to show Jase how it worked, I set up a "beat" for him, as you can see in this video. Then he draws a .. rocking chair. haha.