One option would be to create two workflows. The first workflow would mark change the sales order fields, and possibly check a box or flag the sales order indicating that it has been updated. Next, create a saved search of your child record. This saved search criteria would be filtered by joining to the sales order record, and selecting only sales orders that had been updated (based on user notes, or setting a custom “flag” field as described in the first workflow). Create a second workflow, using the saved search as the criteria. Add an action in the first workflow to initiate the second workflow. This is a high-level overview, but I have done similar things using multiple workflows like this. Let me know if this makes sense, or if you need more details. This is assuming that all you are doing on the child record is setting a flag, or just setting some fields that are independent of the fields set on the purchase order.