So I grew up near a theme park where many parents, older siblings, and eventually friends worked, worked myself as a costumed character in college, and still do stints as non-theme park educational characters every now and then (I probably have the only 5 ft. tall trilobite costume in the world). I survived untraumatized and it turns out that character work is a very nice skill to have.
I don't remember myself or my friends ever believing that the costumed characters were real even though we saw cartoon versions of them on TV all the time. Neither was there a very high level of belief in Santa, the EB, or the TF because adults just didn't see the point of lying when the truth was so obvious to most of us. We didn't suffer too much though and still loved the magic of costumed characters and themed rides. Theme parks can still be wonderful places without 100% of the magic being real. I knew WAY too much about the inner workings of a theme park at too young of an age due to overhearing comments at dinner and play dates, but I had just as much fun as a kid visiting from out of town. It was just a slightly different reality for us and we knew we weren't being lied to. We also learned to keep the magic alive for believers, just as the adults did who worked at the park.
The only time I encountered a traumatized child was on St. Patrick's day at a Catholic school where I worked. The kindergarteners were going on about how leprechauns had left presents for them while they were away from their classroom and one of my second graders was screaming that they were lying and that leprechauns weren't real. I came to discover that the second graders had made a booklet where it clearly stated that leprechauns were mythical creatures of Irish folklore. THAT was a long conversation I wasn't planning on having that day. Lying to kids can backfire and create trust issues for a very long time for some kids.
I doubt those kids at the park that day were traumatized because their parents are probably normally like that and it is obvious at second glance that the characters aren't real. And not waiting in line for autographs let the kids have more time for rides that they would enjoy just as much.
Yeah, the kids are alright...