No harm in giving a very general idea of where eggs and the chicks inside come from. Things not to do: drawings, graphic descriptions and going against the parents wishes. At four his parents should be picking up his clues (the Lowe's comment) that he's ready for some basics.
Chiggers? Your chickens could have them too.
The sick looking bird on here with the head swelling, I would bet a dozen eggs it was chiggers. That's why the ivermectin treatment worked.
I'm so sad that I couldn't come to POOPS, any chance of having it in a more central location?