Mark's approach is definitely what I would recommend. Make sure your multi select choices are coming from a table and not a field dropdown so you can maintain them easily. Secondly The automation should get triggered whenever that field is changed in the modified record option which cycles through and creates a child record for each of the selected item. You can add a step to first delete all existing child records and then add the ones in the multi select checkbox to avoid any duplication. Feel free to reach out if you need help setting up the automation - they are fairly intuitive. Obviously you can use middleware like Workato or Zapier to accomplish it as well.
