DH and I went before we had kids and for two adults - IT WAS SO WORTH IT
The show was spectacular, and while very pricey, we thought it was worthy every penny for a special night for the 2 of us. We always say how glad we were to have gone before DD was born.
I would think with a 7 and 12 year old, you just might not get your bang for the buck. I suppose it all depends on your kids, but as amazing as the show was, and there were many kids there, I just don't think they'd fully appreciate it for the price. I think there are a lot of other things you could do with the $300 that they'd like better. Again, you know your children and their interests best, but we found it to be much more adult oriented. If you are planning on taking them along because you and DW really want to do it, then that's one thing, but if you are going for THEM, I'd wait a little longer.
Another thought would be to put them in a kids club/babysitter and you and dw to go and enjoy (sorry, I'm not sure of the ages of the kids clubs, so I'm not sure if your 12yo would be too old or not).
I hope this helps a little, and I'm sure that others with kids similar ages can be of more help!