I was in the same situation, I was going to be at MK for one night and it was the saturday of president's week and princess half marathon, so I was expecting heavy crowds. It's just myself and my son so it was probably half of what you are saying for your family of 4. I was very concerned about the crowds so we booked it.
I am not really a dessert person and I ate basically 1 chocolate covered strawberry and the kid had a couple of macarons. I don't really find the other desserts at this thing appealing. What I REALLY wanted was the viewing area. We showed up at 6:45 for the dessert party but by around 7 went out to the viewing area to get a front row prime view. This is the view of the castle we had before the fireworks started:
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they shut that lamp off and this is the view during the firworks show:
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I loved the show so much that we changed our after cruise plans from Epcot to MK in the evening. It was a Friday night and still pretty crowded. Instead of going to the dessert party at 6:45, we went to the open area that is the mirror of the plaza viewing area. We showed up there around 6:15 and had our pick of spots right up against the fence. I left the kiddo to hold the spot and went and spent $29 on dinner from Caseys. We had a nice picnic dinner there and chilled out (it was the end of our trip so we were tired and happy to chill for a while). This is the view of the castle before the show:
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and here it is once the fireworks started:
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I would say they were both very very good views and the "free" version only cost me a half hour more of time to secure. That other little garden viewing area didn't even fill up the whole time we were there, but I am "front row" for fireworks kind of a person.