The composite record input accepts sets of records, one set for each parent record. Each set of records must be separated with a blank row (all NULLs). The parent object will not finish processing its record until all composite object children have received a blank row to indicate completion of the composite records. You can use Query transform to prepare child data with the proper blank rows. See also the demonstration video for an example of creating records containing composite data.
For additional information about modelling of composite records in SSIS+, please refer to this article.