What lesson did Elijah's mom teach him after he put the toad in her knitting bag in Elijah of Buxton?
Elijah's mom teaches him not to take advantage of others' fears for his own amusement. She does this by turning the tables on him and giving him a taste of his own medicine.
Elijah's mom absolutely abhors horny toads. Knowing this, Elijah hides one in her sewing basket, and has a good laugh at her expense when she discovers it and freaks out. Elijah's mom does not find his prank funny at all, however, and teaches him a lesson by doing to him the equivalent of what he has done to her.
Elijah despises snakes, pretty much like his mom hates horny toads. Elijah's mom hides a snake in a cookie jar, and when Elijah reaches in to grab a cookie and finds a snake instead, his reaction is just like his mother's reaction when she found the horny toad in her sewing basket. As Elijah runs away screaming, his parents laugh, and Elijah is a little hurt that they are not more sympathetic. Having the tables turned on him makes Elijah realize how it feels to have your fears taken advantage of for the amusement of others. It is likely that, better understanding the ramifications of his actions through experience, Elijah will think twice before he pulls a stunt like that again.