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