Here's what is scary about the Tiki Room:
1. LOUD in places. Not a "we're trying to startle you " loud, just typical Disney "if we have a choice we're cranking the volume up" loud.
The Iago charcter (and Gilbert Gottfried's voice) at loud volume is a little much. Again, not "on purpose" scary, but startling.
The goddess Polynesia - if you are not old enough to get the joke of the song she sings, etc. it would be scary to see her red eyes in an otherwise dark room.
The Tiki Gods and the darkness, brief thunder and lightning, etc. Again, loud but not on purpose scary.
My younger daughter was just on the edge of being terrified when she was small - about this age. She "braved up" and made it through - and fell in love with the show. We saw it again immediately and I think three more times on that trip. To this day (and she is now 15) we must see it at least twice on every trip.
The Tiki Room is easy to see, so my advice would be to send one of you in to see it. If Dad can entertain your little one and you can see it you'll have a good idea of what is coming. Then you can see it as a family and you'll be prepared to hold a hand, cover her eyes, etc. whatever you need to do.
We think it's adorable (and also 15 minutes seated in air conditioning!!!!) so we never miss it. Don't automatically cross it off your list without one of you trying it first.