I think a great intro to RPG for young kids (5-6 year-olds) is Little Wizards. It's originally French, but I have a version in English. It combines enough choice and customization to make it interesting, but without being too complex. It's a world where magic exists, but only kids can use it, so that was a plus for the kids :)
https://www.dicemonkey.net/2020/03/13/rpg-review-little-wiza...