I still haven't beat the main story let alone touch the first quest for the main story but I've been told I predicted the future. Check the dates October, 16 2015. Fallout 4 came out in November, 10 2015.
I'd say that's the premise of the story. I don't think you figuring out the premise of the story means you predicted the entire thing.
Also, even ignoring the fact that you literally said "'plot twist': The child is either a good guy or a bad guy", those are both wrong. The game doesn't explicitly treat him as good or evil, and you can side with or against him.