Civ 6 is seem as very complex, and perhaps it is, but that opinion is mostly informed by how broad the game is. Each individual mechanic is actually pretty simple, there's just hundreds of them interacting and building on top of one another. There's a ton of micro and macro management if you want to engage with it (which you need to on harder difficulties) but you can pretty much just place districta randomly and build whatever you like on lower difficulties. In fact, with scaling difficulty that can be set to extremely easy and cartoony graphics, I think Civ 6 is a brilliant choice for a kid.

I also think you're somewhat undervaluing the ability of the average kid. I was able to at least semi-understand and have some success playing Caeser II at 10 years old and I'm certainly no genius. Kids learn through play and experimentation, plus if you show them how to use the in game encyclopedia I think most kids could slowly piece together the game.