Hi Kimberly,
This is always a tricky question. Although Fantasmic! is, hands down, my favorite show at Walt Disney World, I skipped it when my boys were that age. For one, it's rather late, and I often worried about my children being able to stay awake without being overtired, whiny and grouchy - not a fun experience anytime. Then there is the issue of the show itself - it's all about Mickey's imagination coming to life. While there are silly characters and beautiful princess, there are also villains - lots of villains. Many of these characters are larger than life, and it can get quite loud at times. My children would have been terrified - BUT not all children are.
You know your child best, and you are the best person to make that call. I hate to ruin the surprise for you, but you can get a good idea of what the show is like by going to
YouTube and searching for
Fantasmic Disney World. Make sure that if someone has posted a video in 3 parts that you watch the last 2 parts. The first part alone will not show you anything "scary".
Hope this helps. Have a magical vacation!