I don’t remember seeing the definition of the child/parent in a course before the question 3 in that quiz, so from what I’ve read it in fact means, that the child table has the foreign key which is referenced to some field in other table, which can in fact be primary key there, but it seems that it isn’t the requirement.
One question on SO explaining it in more detail: https://stackoverflow.com/a/29561567/4643681
If someone has other additional info, will be glad if you add something on the topic.
Course: https://www.educative.io/collection/5352985413550080/6335716377231360
Lesson: https://www.educative.io/collection/page/5352985413550080/6335716377231360/6564604328017920