I'd like the ability to share a collection within a section not the whole section itself. For an example, I have a Movies section with a Kids Movies collection in it. I'd like to just share the Kids Movies collection with a couple of child accounts. Same thing with a Kids Shows collection in my TV Shows section.
The shared collections could just show up on connected accounts as individual sections.
Personally, I'd like this more than just a ratings based parental control system because it lets me control things at an individual show/movie level.
A workaround has been suggested a number of times that you can change the rating to whatever works best for you on a particular movie. In other words, if it is rated PG but you feel it is inappropriate for kids under 13, change it to PG-13 or even PG-WORSE. Handling this by the fields that already exist significantly reduce the work required to implement something that could work for most users.
Ratings are there for a reason. If you disagree with the way a movie was rated, change it. It is far easier to deal with the exceptions as opposed to building collections for a library of 2000 movies and 12,000 episodes. This would be much more reasonable if we had "smart" collections like XBMC's smart playlists. I could deal with the exceptions there in an appropriate way. That does not exist in Plex, though.