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Microtransactions aside, if you're trying to protect your kids from creeps online you're gonna have to ban every platform that supports interactions with strangers. This includes several other games you've mentioned in the comments including Minecraft.
Personally, instead of banning it I just play it with my kids on a regular basis. There's plenty of actually decent games on Roblox and it enables game ideas that otherwise wouldn't see the light of day. My favourite is the Ikea survival game.
The problem with Roblox is that they themselves are the creeps exploiting kids
If shooters, beer visuals, and light swearing (think damn, shit, the voicelines are pretty rare and can be disabled with mods) are alright for your kid, check out deep rock galactic. its on steam as well as xbox, and is a 1-4 player coop mining and shooting game where you collect minerals and shoot the ant looking bugs trying to eat you. its incredibly fun and ive sunk over 2500 hours into it without feeling for even a second that any of it was predatory.
Pokemon? Despite many of its flaws, it encourages honest trading amongst friends, it is a classic JRPG, and has no microtransaction. You can play it on emulator if you don't have a switch
Sea of thieves is great, but only plays 4 at any given time. It also has alcohol and some T rated violence, but it depends on what your boundaries are for the kids.
It would be helpful to understand what types of games the kid was playing in the first place to suggest alternatives. I ripped this answer from quora on how to see which games within roblox your child was playing
I went looking for any kind of account tracker and to no one's surprise they are mostly about account value from items, not so much about worlds/experiences they have been on.
Most of the responses I've seen on here are just suggesting games which may or may not be to your kids looking purely based on those games being generally accepted as good.
Was he interested in developing games in Roblox or just playing them? If he was interested in development, consider showing to him Godot. Do a simple tutorial together, maybe a multiplayer that he can send and show off to his friends.
No Man's Sky?
We actually tried it on gamepass and my son was only kind of interested. Thanks for the recommendation though
My brother and I started playing Grounded together as a way to just chill and catch up throughout the week. We were shocked with how much content was actually in the game. It’s like Valheim mixed with Honey I Shrunk The Kids, 4 player online coop, base building, survival. It has a storyline so there’s always a sense of direction and it syncs the world with everyone regardless of who plays so it’s easy to just come and go. I could definitely see a group of 10 year olds getting sucked into this.
What might be a good idea is to try Game Pass for a few months and see what they settle on. Then for Birthdays or Christmas, get them the games they played the most. Not sure how parental controls are like on it, but I hope they exists.
That being said, outside of Nintendo, there aren't many Online games which don't demand their users to pay for cosmetics with fake in game currency. See CTR Nitro Fuelled, Fortnight, Call of Duty, Overwatch and Minecraft skins.
With that said if changing your system isn't an option, Minecraft Bedrock Edition the only game I am familiar with. There is a skin store, but you can't earn in game currency from just playing (from my knowledge). So if they don't have access to the credit card, they won't be tempted, plus the base game has enough options that you can customize your character well enough.
If you can get a switch (and friends have one already), Splatoon 2/3, Mario Kart 8, and Animal Crossing are all friendly non-microtransaction laden games.
I recommend Astroneer - a solo / multiplayer survival game about collecting resources, refining stuff and building up bases on several planets. It's hecking fun in multiplayer and it doesn't have combat. There's a ton of things to do and if the kids are good at communicating with each other they can quickly conquer the game. The only microtransactiony stuff it has is cosmetics (but it's not being pushed upon you in any way), but you can unlock some of these through progression as well.