It's not a game, it's a child exploitation engine.

Can you please go into the specifics of why you don't let your kids play Roblox?

I had been avoiding it until a well intentioned relative got my son a Roblox gift card.
It was immediately made clear that I was against the game which created an awkward situation for my relative.
I decided to soften my stance, and use it as a teaching moment.
I explained how the game works and how my son was to use his Robux.
He was to play the game(s) for a certain length of time and then make an informed decision about where to spend his money at.
Of course he spent it on some stupid powerup on a game that he ended up stopping playing some days later.
We used it as a teaching moment for both of my kids and what a waste of money the game is.
I also explained to them how P2W games are predatory in nature, and that I would never personally allow them to pay a single cent into Roblox or any other game.
My son is constantly proud of how he is F2P and always beating all of the P2W noobs.

I used to play it with my nibling. I had no idea it was so awful.