Also I don’t think it’s kid friendly but the guys who made the dungenorator are working on this game that I’ve backed:
Which uses poly dice to represent the different heroes. Maybe for just a bit older? Then again I was playing MTG as a little one so who knows how quickly kids develop now adays.
Dungeon Hero is likely a great start. The same designer has a similar system for a paper craft dungeon crawler called Fable on your Table - it is near the top of my To Try list, and looks like an incredible starting package, with the most complicated part of the paper crafting (cool door frames) essentially optional.
I just a few days ago got a message from someone, that their 9 year-old cannot stop playing my game Dungeons of Doria. Playtests with interested children around me indicated that from 8 or 9 is a good age, even though we indicated 14+ on the box. Especially if your kids speaking natively english and are interested in D&D likes, that should work.
It is close enough to D&D and cooperative, though more focused on combat encounters and not a lot of RPG in it. But might be worth it. Descent, Bardsung and some other board games also have polyhedral dice, though often not numbered and with special symbols.