The action to add multiple rows is the Create Tasks action isnt it ?
Option 1; Simply place this action either place, overlay, permanent. User explicitely hit action button to add row.
Option 2 ; Go to form view where the action is starting from. For setting of form view, place this action to fire “on save” to fire action. But problem with this set up is action will keep fired all the time when the form is updated.
Option 3 ; Fire this action through the workflow, ideally ADD ROW event to the table. Once the new row is created and saved, the workflow is fired and action will add rows to the child.