If something is removed from a concept, remove it from the exhibition.

In the visibility settings, the owner chooses how future children should be treated (default to not being shown). But if they choose to show them / show meta / show preview / allow download, change the visibility information during a tree change.

These settings are available from the exhibition visibility information.

make a hook in exhibition_config for islandora for when the RELS-EXT is updated
If a concept has gained more resource children apply the "future children" to the new resources
If a concept has gained more concept children apply the "future children" to it and add the "future children" settings to the new concept
resave the exhibition visibility