watch this
youtube video yourself of the whole show --best color and resolution, but it just shows the dark screen when the spiders come down
http://www.youtube.com/watch?v=wrairzx37gc&feature=related
then watch this one, which is much grainier, but will give you the total impression of hte bug spray and the black widows coming down:
http://www.youtube.com/watch?v=oQDN74pvbRk
(copy and paste the URLs into your browser if the link isn't working)
that should give you a good idea of your child's tolerance. It is more intense in person than the second video can convey because of all the screaming and little kids freaking out. When we saw it when my DD was 3 (her brother was 10, so we were balancing 2 generations) we sat on the very end near the exit, and i had her on my lap.
She knew what was coming, and screamed, but was okay afterwards, and was determined to "conquer" it at age 5 when we went back. Her brother made her a sticker page in her autograph book where she got stickers when she "got" the "scary' attractions ... ToT, Dinosaur, ITTBAG, HISTA, Snow White, etc. He promised her a beanie from his money when she "got" them all. It worked for her.