Welcome, bonek.
Note that each of the existing filters is based on a particular attribute the program can ascertain. Adding another filter would not be a simple change, so this is unlikely to happen soon. And if it is to be done at all, it should probably be something more than one just based on a specified flag (like Wish). My hope is that someday we'll be able to save Advanced searches. When that feature is available, it would be wonderful if a new filter could be based on one of those saved searches. Then you would be able to devise a complex search to identify the movies you don't want your kids watching, and filter them out.
In the meantime, consider using the loan filter to hide movies you don't want displayed—for whatever reason. Just select all the movies in question (you might use Advanced search to do this), and then "loan" them. Enter a borrower's name that describes why they are being tagged for hiding. You will then be able to use a simple search of Borrower to see what movies are in each of these categories. I make good use of this technique to hide concerts, documentaries and shorts—so my list is restricted to movies.