I just started tinkering with Godot, seems very, very good and the licensing means you won't suddenly have the rug pulled out from under you.
I'm torn though, the recommendation for things like Roblox seem valid, but indeed it seems like the kind of thing where you will hit a glass ceiling. On the flip side getting started on something more generic may be tedious and cause an 11 year old to lose interest quickly.
I think it comes down to the motivations and nature of the individual - do they wanna learn the skill, or do they want to create something quick that they can show off to their friends? (and maybe go further if they really enjoy it)
Simpler game systems weren't flexible enough to capture his imagination. We also tried to get started with Unity, but we couldn't get past the first steps.
Godot if you want coding. Coding is required but it uses gdscript which is a python inspired language. The editor is also batteries included, so godot.exe will include everything you need to make a game (aside from assets, but even there some plugins can help). 2D and 3D, but 3D is not "AAA" tier.
I honestly recommend you go for godot. There are tons of tutorials out there to create all kinds of games, specially if it's something basic. It's what I wish existed when I was 11 and had tons of time to mess around.
I'd say look up a quickstart tutorial for both and check if your kid likes any of them.
I see lots of recommendations for niche/toy engines. I don't recommend those because it'll be hard to look up tutorials for them. Having guidance in essential. I also don't recommend your kid get to roblox, sure he might be able to make some money or get lots of attention, but having external motivators can be very detrimental.