Why are the child records not displaying in the parent record form?

If you can no longer see child records in a report that you've embedded on a parent form, someone has changed the Filtering (matching criteria) for that report.

Check settings of the embedded report - particularly settings in the Filtering section. Reports you embed on a parent form should have no filtering criteria set. More details about adjusting embedded reports can be found here (specifically, see the FAQ at the bottom for more info).