I will be honest Fantasmic is something to see, but it can be scary for children. My children were 9,6, and 4 when they saw it for the first time. While my 9 yr old liked it, my 6 and 4 year olds hid their faces for most of it. It is loud, and there is a scene where all of the Disney villians are presented and they kind of progress and transform from one to another. There is evil laughing and cackling going on, and shadowed horsemen appear. This part of the show scared my childen worse than the huge dragon. From my observations, most of the younger children around us were crying and hiding their faces too.
It is a hard call to make. If your children are not timid, then I would try it. It is someting to see. Just be warned that your children might not like it.