Rainsong,
There are a number of possible scenarios, some innocent and some evil. Here's one of them:
It was Christmas, and the boys had a lot of new toys to show off, but no time to do it. Burke had spent some of the afternoon and most of the evening at the White's boorish dinner party. And the Ramseys were leaving early the next morning for about 10 days of wintering at their Charlevoix getaway and then on to Florida for a voyage on the Disney Big Red Boat. The boys didn't have a chance to be together.
The young guest that night (in my theory) could have been invited by the parents, or he could have been snuck in by Burke and JonBenet after the parents had gone to bed. I tend to believe the latter because the security light on the corner of the house had been turned off and the bowl of pineapple and the tea glass on the table suggests Burke and JonBenet were waiting for someone to show up. He would be the fifth person in the house that night, and perhaps the killer of JonBenet.
On the other hand, had the guest been invited by the Ramseys, then a baby sitter would have likely accompanied him so the children could stay up late and play with their new toys under adult supervision while the parents got to go to bed early. They would be the fifth and sixth persons in the house that night, and the adult babysitter would likely be the killer of JonBenet.
BlueCrab