It would be helpful if there could be a warning or indication if subitems have not be acted on and the item is going to be closed, this would help prevent people from forgetting to check subitems.
Bianca’s solution works really well! You can add notifications so that the person responsible for the item are made aware.
If you want a visual way of seeing the subitems have not been completed on an item, you can also add the Summary of the subitems’ status on the parent item.