This is actually expected functionality, as parent tasks are not allowed to have resources. Similarly, child tasks must have resources removed before adding a new child task. The reason for this is that only the lowest level tasks are allowed to have resources, which prevents them from having children.
If a child is added to that task, then it is no longer the lowest level and resources are not allowed to be assigned. Since parent tasks cannot have resources, there is no way to report on this. However, you are able to report on all non-parent tasks by creating a Tasks Report in the Analysis application that includes a Parent? is false filter.
I hope this explains why parent tasks are unable to have resources, and an alternative for reporting on only non-parent tasks. Please let us know if you would like further clarification or there is anything else we can do to help you.