I wouldn’t call Roblox itself a gacha game. That category is the ones where you are trying to collect all the heroes in the game and level them up with rare loot. AFAIK they generally, if not always, involve loot crates that you have to purchase.
Roblox has its own problems. As spelled out by People Make Games in these two videos.
Roblox claims to teach kids how to make their own games. At this point from what I’ve heard, I would suggest Unity Engine before Roblox, and I wouldn’t recommend Unity after their pricing debacle.
Watch the videos, and have a serious discussion with his parents about it before you get him that game platform.