Not a bad idea and I'd probably go ahead and just change it, unfortunately there's currently 292 Geared for Children stories and you can't just change the title of the tag. What would happen is the tag would be deleted and then all 292 stories would have to manually get labeled Family Friendly/All Ages.
Though I'm likely to just go ahead and do it anyway and just bother to relabel Imagination (And maybe a few of the others I actually remember have the Children's tag) with the new tag.
Yeah let's go with Family Friendly. I like the alliteration and it's less cumbersome.