Maybe it would help to say it another way? First the phrase "children carried forward" means those children who will be repeated (carried forward) as the Subject of their own separate Journal section later in the report. But children get their own later separate section only if they in turn have children. That separate Journal section will include all the tags specified to output for the Subject of a person's own section. But if a child does NOT have their own children, a separate Journal section will NOT be produced later for that child. (That is the definition of a Journal report.) The only place child-less children and their events will be output in a Journal report is within their parent's list of children.
The typical setting for the Report Option "abbreviated events" is "Only children that are carried forward" (i.e. children with children). Since these children will be repeated (carried forward) in their own Journal section in the next generation where they are the Subject of their own section, most people choose to have only "abbreviated events" in their parent's children section to avoid duplicating the output of all the event tags for this child in both the parent's children section and in that child's own section.
But most users do not choose "All children" because that also would abbreviate the events for those children who are not carried forward. Since the parent's list is the only place events for such children will be output, any events for such a child other than the specified abbreviated events will never be output in the Journal report with that setting.
As Terry suggests, some users will choose "All children" to avoid the output from lots of events for all children who have no children. But that is usually a special case.
Of course if the number of generations is limited and this is the final generation, the children of this last generation are a special case since their separate section would be in the "next" generation which will not output. Thus such children will not be "carried forward" to get their own section with all their events, whether or not they have their own children, so their events will output within the parent's list of children as specified for children not carried forward.
Does this help?