This situation would best be described as a teachable moment.
Description of a teachable moment
A teachable moment is when there is a chance to learn something important. It is when someone, usually a kid, learns something that can be used to teach and help them.
In this situation, when the child had a strong reaction to the broken crayons, it became a good opportunity to teach them something. The child forgot to pick up the crayons, so when the younger sibling came along, some of them got damaged or lost.
When your child cried, it gave you a chance to comfort and understand their feelings.