If your child is having a lot of difficulty double-clicking with the mouse, you may want to adjust the double-click speed settings. This may be necessary until your child becomes more proficient at using the mouse. Here's how to do this:
Directions on how to lower the double-click speed on Windows can be found in this article:
How do I lower the mouse double-click speed in Windows? (KB000011)
Directions on how to lower the double-click speed on Mac OS X can be found in this article:
How do I lower the double-click speed in Mac OS X? (KB000010)