I was hoping to find a better screen shot (already available in my Photobucket account), however I think this one will do for starting out. I can upload some better screen shots if needed.
I'm not sure how you got the screen layout where you are now - but, basically this is what to do:
Note that my screen shot is not the MC default. I have made some changes but the steps below are the same.
First
1) Go to Tools>Options>Theater View.
2) Note that the items listed in column "Items to Show" is how the
Top Roller looks when you start Theater View. Those Items "Left Justified" (like Audio, Video, DVD, YouTube, etc.) will be on the Top Roller. In the screen shot you can only see that Video is left justified. BTW - you can move the display order of the Roller too (with the move button and nest or unest Roller items here).
3) Then the items indented under those "Left Justified" items are what shows when you click on the item on the first Roller. So if you click on Video (on the Top Roller) that opens the Roller for "Movies, Shows, Guide" (which is the
Second Roller).
Sorry if this is too basic.
4) So in my screen shot I have added a new selection called History (It could be Kids Movies or whatever you want). You use the "Add" button to add a new item (in my example, click on "Video" to select that level, then use the "Add" button and select add a "Library Item"). The "New Library item" is now grouped under Video. If you wanted it under "Movies" you would click "Movies" then Add the item at the Movies level.
You would rename "New Library Item" to "History" (in my case) in the Details window.
5) Now you need to setup the new item's (History) display rules. So click on "Set rules for file display..." button. That bring you to the screen shown below
6) The Rules will be blank, so Click the drop down arrow and find "Media Type".
7) Media Type is "Video".
8 ) Set a sort order (can be sort by name, date, year, etc.)
Think about this and post back if I'm not clear, or you want more help, or if I'm just messing you up worse.