Hi,
If you are always creating a child case from the parent case and you have a 1:N relationship to the child cases, you can use the 'Mappings' section in the relationship customisations. This allows you to map data from the parent to the child so that when users create a new record, the data will be pre-populated.
This data will be a static copy so that if the parent data changes, the child records will not.
hth,
Scott