>The perp would have had to observe the family at some point to know about little JB.

Not really.
It is likely though.
Draft of note: We have your (son/daughter) in our possession.

Now, quite frankly, I think he knew about her in advance. And I also think there most likely was some 'observation' of the premises. Maybe prolonged such as by a neighbor or maybe fairly briefly such as from a distant vehicle, etc.

I think he saw them troop out on their way to FWs party, simply because no one has been identified as the person who was seen approaching the house in a non-furtive manner.

If the two year old article with red ink is related to the crime, that might have biographical material. If not, it certainly shows he knows how to obtain such information. I feel confident he selected who he was going to kill in advance. I just don't think JonBenet was the real target of his crime. She was just an enjoyable means to an end: creating pure hell for the parents.