I don't think it's too scary. The graphic isn't grotesque, and any kid old enough to take the time to read the text is probably old enough to get that it isn't really going to happen.
It's perfectly fine if you like the shirt. The majority of people are dressed with more whimsical things but there's nothing whatsoever wrong with the shirt for the party (or during the day either . . .). You'll see a decent number of people with more "dark" clothing.
As a mom of 3, I don't think it's too scary. Just so you're aware though, the theme of the Halloween stuff at Disney tends to be much more cute-spooky than scares, so you might be disappointed if you're expecting a haunted house type vibe.
Definitely would at HNN I know they had a trick or treat house and/or scare zone a few years ago…. Don’t personally do “haunts” but love seeing pro-shots of them on YouTube lol