Hi Pamela,
The evenings in Walt Disney World are magical. I love the change in lighting and the cooling of the evening. I also think the pixie dust is extra special.
While I love the evening shows such as fireworks at Magic Kingdom,
Fantasmic!,
Illuminations: Reflection of Earth and
Rivers of Light, the real question is how your children will handle going to bed late. For my children at that age, they loved being at the park early and heading back to the room and being in bed most nights before these events took place. If we did go to a show in the evening, we made sure the next day was a later entry into the parks, visiting a water park or hanging at our resort pool. Even now, the girls (18 and 20) would rather not be out late every night and be up early for the parks.
If your children will not have an issue, then go and enjoy. Otherwise, you can always come back and see these on another trip.
I will say Magic Kingdom fireworks are best viewed from the beaches of Disney's Polynesian Village Resort.
Have a magical vacation!
Beth