The earlier you show up, the better choices of seating you have. I booked an early dinner, like 4:30-5pm and then went to the 8pm show. And we got in like like an hour or so before hand. That was our christmas day, and that was the only thing we were there for. So it didn't bother me waiting for a while.
Last year we had an amazing time at Garden Grill. It was THE BEST character experience ever. Dale just made it completely worth it. And the fact that if you book the package dinner, it's not much more than regular dinner price.
Just remember if you want the 5pm show, you have to have lunch. The later two shows allow for dinner seating. Just remember the 8 oclock show lets out about 15-20 minutes prior to the fireworks, so you might have a harder time finding prime seating for that.
What are your dates for this?? The closer to christmas the harder it gets for getting reservations, restaurant you want, finding a spot for the fireworks. The earlier in Dec you are, the easier it is and less time you have to wait.