Word of warning about Roblox, it is home to child labor exploitation, gambling and chat rooms rife with predatory behavior. They allow young game developers to sell their creations and take a 70% cut of their revenue; that is if they let you cash out at all. I recommend caution and at the very least be involved in what your children are doing in Roblox.
Seconded on the money part. I'd treat it as just a game engine to scratch the itch of learning and steer them towards actual programming in Lua.
Then, even if the language would be different, they can transfer to a proper game engine like Unreal.
But publishing the game in Roblox for real could still be a good thing (or not). Just don't expect to "get rich fast". It can teach that even you have a cool game, nobody is gonna find it and you'd have to stand out. In the end it's a bit like YouTube. Same exploitative relationship. You might make money. Most won't. Many will feel lots of pressure for no reward. And even with reward the pressure might become too much.
