Tom means a Reference Attribute Shortcuts which allow you to display reference data in a grid or report. The easy way to think of it is a quasi JOIN. There is a detailed description in the User Guide.
However, I am wondering if you are looking for reports which have both parent and child data (literally in this case which adds to the confusion! lol) on the same layout? i.e.
Child 1 (parent record in data)
- Parent 1 (child records in data)
- Parent 1
- Parent 3 (?)
Child 2
If so I think you need to experiment with Sub-reports.
For your example, if you just want a list of all child (records) where the parent (record) meets a specific criteria without including the parent record on the report (like for a form letter mailing list to all parent where the child is under 18 or something like that) then you would create a query like...
'Parents of children under 18' with the query as Parent.ChildReference.Age<18
Then use that as that query as the selection criteria in the when you set up you document.
Is that what you are after?