Multicategory Sorting

I know that you can call or sort a post by the category. I need to call and sort my posts by 2 subcategories. eg. I have a parent category called games with two children categories called water games and beach games. I then have another parent category called age group. This has two children as well called 10-12 years and 13-16 years. I need to show the posts that fall under water games and 13-16 years. Is there a plug in or what is the code to call 1 post with muliple categories selected.

thanks