That is exactly where I expected to find it, however it was not there.
I found the reason for that option not being available, the parent forum was set to type "Forum" rather than type "Category". Apparently if the parent is an actual Forum, then the subforums cannot be listed on the index.
From what I can tell, in order to have the sub forum indexed on the front page you need to first make the parent forum a category and then go in and check off the index option on the sub forum. You then have to go back in and make the parent forum a forum again or else no one can post in the parent forum because it's a category.
Once you make the parent forum a forum again the index option once again disappears from the sub forum edit page. If you want to remove the sub forum from the index page you must once again make the parent forum a category remove the index option from the sub and then make the parent a forum again.
Sounds ridiculous, but that's what I'm getting.