Let's face it, kids have a notorious problem: they're short. But they usually don't stay short for too long. So if you build a bathroom cabinet for a kid at their current height, then it's too short when they get big. If you build it for their future height, then you're dealing with step stools cluttering the floor.
One of the benefits of custom cabinetry is we can build it any way we like. One of the tricks up our sleeves is to utilize the space under the sink. If you plan the plumbing properly, you can use the space below the sink as a hidden drawer, or, in this case, a built-in step stool.
Once the kid grows out of his or her inconvenient shortness, simply convert the step stool to a drawer, and you have very clever storage. ;)