Are you really suggesting that Roblox's cut should be lower purely because the target market is children? Why? If anything, the fact that a kid can code a game in a high-level environment and immediately start making money—without any of the normal challenges of setting up infrastructure, let alone marketing and discovery—is amazing, and a feat for which Roblox should definitely be rewarded.

In any case, what's the alternative? To teach your son how to build the game from scratch in Unity, spin up a server infrastructure that won't crumble with more than a few concurrent players (not to mention the cash required for this), figure out distribution, and then actually get people to find and play the game? That seems quite unreasonable for most children/parents.

If this were easy, a competitor would have come in and offered the same service with significantly lower fees.