So I'm reading The Adventures of Tom Sawyer to Wolfie and Xavier (the ds10 formerly known as Chester) at bedtime. In the second chapter (the whitewashing the fence chapter), both boys were entirely mystified first by the elaborate description of Tom's friend, Ben, pretending he was a paddleboat tug, and then by the various "treasures" the boys paid Tom for the privilege of whitewashing the fence. It's possible that because they had never seen a paddleboat before, that they were unable to picture in their minds what Ben was doing. And the huge numbers of toys available to the modern boy make a one-eyed kitten, a piece of broken glass and "a dead rat and a string to swing it with" seem of dubious entertainment value. Or maybe they're just so coddled that they don't know how to play.
No, that can't be the problem. Before they went to bed, Xavier sent a "wolf-gram" to Wolfie with secret directions in it and Wolfie responded with a full-fledged treasure map delivered by his "identical twin brother who didn't have a pencil taped to his shirt". (And it turns out the "X" on the map marked a real "X" Xavier was supposed to find. It was two pencils taped to Wolfie's shirt at right angles.)
So I don't think the problem is that they don't know how to play. They're just downright weird.