Have a 12 and 13 yr old. 12 yr old is autistic and creative and has no experience and 13 yr old is imaginative beyond all else and has minimal experience.  Can anyone suggest a couple of good free starters to get them going. They have they basic starter set already and have a quick version Adventure Begins game.

I am not sure which starter set you have, but the Essentials Kit which has Dragon of Icespire Peak is really good for kids and has rules for sidekicks. They can treak the sidekicks like Pokemon or a pet if you make them a non-humanoid. 

The essentials kit is set up more like how a RPG video game works with quest hubs so if they have played a video game with missions it will feel similar and easier to pick up. 

Though the Essentials Kit isn't free you get a lot of content and rules for playing with just the two of them as players for about $20 - $25. It can easily provide a gaming experience for a year if you also use the digital downloads that come with it. 

Feel free to reach out on my Discord if you have any questions about it. One of my friends just ran this for his kids.