How do I get the weight of a multiple value field collection in a computed field?
Ok, finally I got a workaround. I was focussed on entities and computed field, but it turned out to be a death end. So, I change the approach and found the rules_form module.

This modules allows apply a rule in the node form, so I created an integer field called delta in my field_collection_parent and populate it with rules when the form is created. In this way, I always keep a seven-rows field_collection_parent even though the field_collection_child is empty (because the field delta is filled).