I used the Black Hack (a really stripped down version of D&D), with this version of Matt Colville's The Delian Tomb: https://www.gmbinder.com/share/-L_0WX-KKXsZA22VHr5j - with my kids (age 10 and 12) and it was a blast. Opportunities for roleplaying at the village and combat at the tomb if they want to hit things, although my kids wanted to trick the goblins to let them past (which they did, hoping the PC would get hit by the trap further in).

Black Hack is great, roll up characters very quickly and you mostly just use the attributes, so if something requires a jump, they have to roll under whatever their DEX is on a d20. Really simple to get your head round, no mucking about with skills. Or you could just google D&D microlite and finding something that suits (also some of the illustrations might be unsuitable, thinking about it).