What seems rather strange about the description of the child playing the flute?

The child playing the wooden flute is described as sitting "at the edge of the crowd, alone." His isolation is thus the first part of the description that we might regard as strange.
The people who stop to listen to the child never speak to him because "he never ceases playing and never sees them." This also seems strange. It doesn't seem possible that one could play an instrument without ever stopping, especially a woodwind instrument which requires controlled breathing. It is also strange that the boy never sees them. This detail gives the impression that the boy is, figuratively, in his own world, completely unaware of the people passing by. The boy's eyes are then described as "dark" and "wholly rapt in the sweet, thin magic of the flute."
This final part of the description contributes to the overall impression that this boy is perhaps a mythical, magical creature. Indeed, the music he plays is described as a sort of "magic," and the word "rapt" also suggests that the music is enchanting.