Your little brother is not spending money in Roblox. He is spending Robux. That difference is the entire point.
Most big games do not take your cash directly. You buy an in-game currency first, Robux in Roblox, V-Bucks in Fortnite, and then you spend that. The extra step matters more than it looks. Once real money becomes "points," it stops feeling like money. You spend more freely, and it is much harder to track what you actually spent, especially for a young player. In Roblox, players buy Robux with real money. Creators who earn Robux can cash some out, but at a far lower rate than players paid in. Roblox and the app stores keep most of the value in between.
Translation: The currency is a buffer between you and your bank balance. Ten pounds feels like ten pounds. Eight hundred Robux feels like a number in a game. The game is designed so it feels like the second one.
Translation: Roblox is a public company. Those Robux purchases, made largely by young players, are its actual revenue.