BeatMaker wasn't created for a client - I developed this little Flash application a couple years ago to help a friend's five-year-old daughter, who was then starting to play the drums, understand the basics of rhythm and counting time.
The interface is simple - the screen contains on measure of music, broken down into eighth notes, with three instruments - closed hi hat, snare drum and bass drum. It was tempting to add more drum sounds, but the idea here was to allow simple beats to be created - not to make a virtual drum machine or anything so complex that it would get in the way of learning.
Each note is clickable - clicking toggles the note on or off. There are four speeds ("Slow", "Medium", "Fast", and "Crazy Fast") and there's a voice (my own) that counts each note in the measure. It can be turned off so the child can try counting on their own. A vertical line scrolls in sync with the counter, so kids can connect the counting to the current position in the measure.
I've included sixteen sample beats to help give kids an idea of how each note can be arranged. There's also a Clear function so they can wipe the slate clear and start from scratch (rather than manually turning each note on or off), a Random feature that creates a totally random beat, and there's a Print function so favorite beats can be saved and practiced later - even if there's not a computer around.
That's it - nice and simple. Check out BeatMaker.