Media subtype affects, for example, the get TV and Movie metadata operation. If it's marked as a TV show and there's no season or episode, it will error, but not if it's a movie, etc. There are other places where media sub-type matters for programmatic reasons. The inclusion of "Adult" as a media subtype is a bit confusing, unless it needs special programmatic handling.
I'm -1 on adding children's anything as a media subtype.
FWIW I use a children's genre to filter all my views, it's trivial to do, and I can't imagine why it would be any easier to use media subtype (adding a tag is adding a tag), and genre is a list type tag.