You could do the reverse and label everything you don’t want your kid to see, and exclude that label. But I suspect that’s not much different.
Are you aware that you can add labels to many movies at once? You don’t have to do it one at a time. If that’s why you want to find a way around this, you may not have to.
Instead of using labels, you might create a Playlist of the movies you want your kid to see, and share the Playlist, rather than the library.
