I think Kensington makes one for the Mac with only one button. They used to sell Mac mouses from way back. However, I think that mostof the mouses they sell now have two buttons, but I could be wrong.
To be honest, it shouldn't be that hard. My son has been able to use the Apple mouse without any problems and he's 4. Not that every kid should be able to do this, but it's possible.
What I did at first is to get him used to a trackball mouse. He was able to move the mouse around using the trackball and then pushing the buttons above to click on something. Once he understood the concept, he eventually moved on to a mouse and got quite adept at it.