First, make PINs visible in the Parent Portal, then you can direct them there! Go to Tools > Parent Portal > Settings and checkmark Show Contact PINs. You can then send out an email to everyone and include a link to your Portal login page. Once they login, their PIN is visible on the My Contacts page.
To tell them in person, print a PIN Report from under the Clock menu in your database, or print a Contact Listing Report - that output includes PINs and Pickups.