1st script - Need help to update a customer parent field when a child record changes

First time creating a script.  Could really use some help.  I have included my script below. Thanks in advance for any help!!!

We have a custom entity field called Last Interaction (date field) that we want to update based on:  1) If a child Work Order changes status to either Revenue Released or Work Completed and the Event Date field is populated and newer than what is currently in the Last Interaction field then set Last Interaction to be the Event Date (since it is newer). 

The Work Order could either be created with both of these criteria being met and of course, edited at any later date to satisfy the criteria as well.  The script will run on the Work Order record on the afterSubmit operation type.  I plan on doing another script for Events but that one will be easier and almost the same, once I get this one figured out.