If you will have a car you could take them to Medieval Times. The outside looks like a castle and the show itself would probably thrill them way more than eating in Cinderella's castle.
When you eat dinner in Cinderella's castle they don't really do anything special compared to any other Disney restaurant - except the CM's call the children "princess" and "prince". It's not a character meal. If you can get a window seat it's kind of neat to look out over part of the park or watch the fireworks. However, you get a much better view of the fireworks if you watch from outside and, if you do have a window seat, other diners will come crowd around your window when the fireworks are on.
I'm not trying to say it's not worth eating there - it is kind of cool to be inside the castle - but IMO it's not worth any great despair if you can't get in. There are lots of other restaurants - Disney or off-site - that have interesting theming.