I know this is an old post, but I will give a +1 to Hero Kids for a few reasons:

• There's like 4 stats and all you need is a bunch of D6.

• The character sheets fit on an index card and are easy enough for a 6 year old to grasp everything on it.

• If you want to run their setting, tons of info is provided with quest seeds and pre-made adventures, and it is super easy to adapt to a different setting, or even totally different genre, because of how simple the stats are.

• It has a great monster compendium, with tons of fantasy monsters, with notes on balancing encounters.

• If you want to get creative and make totally custom monsters, the information included on balancing gives you all the tools to do that.

• The PDF bundle on DriveThruRPG is like $20 for the PDFs of most of the adventures, the core book, the monster compendium, and the gazetteer, all of which are great.

I'm running an ongoing campaign with a 6 year old, a 7.5 year old and my 16 year old sister. We had a 3 hour session last time, and the ADHD 6 year old was there for the whole thing (with a couple 5 minute breaks).

If you have any questions about it let me know. I'm not a super experienced GM. I've only run Hero Kids and Genesys, with being a GM in Hero Kids being my intro to TTRPGs.