Dennis @ PerfectForms
The child form is a different datasource. You can have more than one datasource per report page, but they will not respect the dynamic filtering of objects reporting on different datasources.
I recommend that you use a form object in the report. For example you can use a drop-down or a table and populate it using a form connection. When you choose an item from the drop-down use a set filter behavior object for each datasource on the page. Filter by the value in the form object.