I would recommend A Midsummer Night Dream for a starter. I do not own either book, but I’m sure one of them has it in there. It is a fun, fanciful story.
I’ve always loved Shakespeare, but I know he’s not everyone’s cup of tea. I’ve decided, as it is my first first year as well, to put off starting Shakespeare until next year when my boys are in 3rd and 6th, and that is the one I will start with.
Another interesting thought (though I’m sure many won’t agree) would be if there is an age appropriate re-teeling of Romeo & Juliet. You could watch the movie Gnomeo and Juliet and ask your children to look for similarities in the stories. (Just thinking aloud).