You'd be getting out of BOG by about 7:30-7:45 if you are quick. You should make it to the dessert party in about 10 minutes. So, yes, you'd miss out a little on the desserts, but the dessert parties, at least to my knowledge, are ok with guests being a little late.
My main concern is how hungry will your family be for the dessert party itself? The portions at BOG dinner are huge! My family has 3 teenagers, and even then, the five of us split only three meals there. We were barely able to finish all the entrees. After remembering that feeling, and then thinking about eating at a dessert party 10 minutes after is making me queasy.
I understand you want to go just for the view, and that's alright. But thinking about paying $60 a person for noshing on basically 3 small desserts (I can't think of eating more than that without being sick) just doesn't sound like a good time.
Another example from my upcoming trip: I have a reservation for Le Cellier which was originally at 5:30. Then we thought about hitting R&C or FEA dessert party near 8 to watch Illuminations. Even after thinking this through, we thought that this was too close to each other. So I was able to move Le Cellier to 4:45 to give us more time to empty our stomachs.