Well, as a start, it is hard to tell what actual rule you used based on your post above. If you need help, always copy paste the search from the Import/Export part of the Search Wizard into the forum, so we can see what you really searched for. Because whether you're doing substring searches or have a mistake in there is hard to tell.
But, more importantly, I wouldn't use [Media Sub Type] Home Video to accomplish this goal. Home Video has special handling in Media Center, I believe, but in any case, there's no reason to use a nonsensical "magic value" that doesn't actually apply. In fact, I probably wouldn't use [Media Sub Type] at all. If you want to use [Media Sub Type], I'd use the Adult value that is available there.
But restricting the Kids view by [Genre] would probably be the better way to go for your use case. The easy way would be to just tag them all with a value you invent for this purpose, like: -[Genre]=[Not For Kids]
Files can have multiple Genres, so this doesn't even interfere with your ability to browse these (when the Kids user is NOT activated) by Genre if you want.
But you could just collect actual Genres (like those that get auto-tagged from TVDB and TMDB) and make your rule something like:
-[Genre]=[Adult],[Crime],[Disaster],[Horror],[Mafia],[Suspense],[Thriller],[War],[Not For Kids]
That way, things that come in pre-tagged with one of those "pretty obviously not for kids" genres will automatically get excluded. And you'll only need to manually apply the special [Not For Kids] Genre to things that get otherwise missed. Now, keep in mind, those exclusions will apply across the entire Library. So anything that has a matched value in the Genre tag (including, for example, music or audiobooks or TV Guide entries) will get excluded too.
You can prevent this too by making your search more specific. The easiest way to do this would probably be to use a separate smartlist and then exclude your User from seeing the files in the smartlist.
Make a new Smartlist somewhere (I have a Smartlist Group called "View Filters" I use for this kind of purpose) called something like "Kids Exclusion List". Then, give it this rule:
[Media Type]=[Video] ([Media Sub Type]=[Adult] or [Genre]=[Adult],[Crime],[Disaster],[Horror],[Mafia],[Suspense],[Thriller],[War],[Not For Kids])
You could even get fancier and exclude items based on Ratings too, and throw in the Image Media Type if you have any images in there that might be inappropriate for children (I'm not judging):
[Media Type]=[Video],[Image] ([Media Sub Type]=[Adult] or [Genre]=[Crime],[Disaster],[Horror],[Mafia],[Suspense],[Thriller],[War],[Adult] or [MPAA Rating]=[NR],[R],[Unrated] or [Access Rating]=[TV/-MA])
The good thing is, since the list is limited to only [Media Type]=[Video] (or maybe Images too) won't "accidentally" trigger on any TV Guide entries (which are [Media Type]=[TV]). Using a separate Smartlist is the easiest way to accomplish that because the search filters "down". Plus, that list will now show all of the stuff you want to NOT show up in the kid's View, which will make it easier to manage and tweak the search as needed.
Then, to actually exclude these files from the Kids user (without matching anything else that happens to intersect) open up the Kids User search and enter:
-playlist="Kids Exclusion List"
Now, the Kids user will exclude anything in that specific Smartlist, and only the results of that specific smartlist.
By the way, type this value in manually into Import/Export when editing the search for the user. If you use the Playlist Is Not Any selector in the Wizard, it will convert it into a PlaylistID link. These, however, can get broken if you use MC in a Media Network environment (PlaylistIDs can change "behind the scenes"), so don't use it that way. Instead, use the -playlist Rule which searches based on the playlist's name. That is a bit slower (I think) but much more reliable.