Parent, Children and Cascade deletions
"Children" elements are elements that directly depend on another object (the "parent").
They only have sense in a certain context, and if the context to which they belong is removed, to mantain data meaning and integrity the application provides also the removal of them (now become "orphans").
Example
Take the case of a Travel and of the Events occurred during it.
In this example, deleting a Travel will leave all of its Events meaningless since, taken alone, they would have no context in which to place themselves, nor a way to reach them.
Consequently, the removal of a Travel also causes the elimination - cascading - of all Events that occurred in that Trip.
Important
Note that this operation is not limited to one level of relationship, but proceeds with children of deleted children and so on, until the entire database is freed from unnecessary data.
This lets the user remove all data about a Travel by simply deleting that Travel.