I don't think anyone objects to adults making that choice over say, using Unity or Unreal, and targeting other platforms.
In practice, explaining to my son who is growing into an avid developer why I won't a) help him build on Roblox, or b) fund his objectives of advertising and promoting his work in Roblox (by spending Roblox company scrip) on the platform has necessitated helping him to learn and understand what exploitation means and how to recognize it.
It's a learning experience for him, and a challenging issue for me as a technically proficient and financially literate parent who actually owns and run businesses related to intellectual property. It's got to be much more painful for parents who lack in any of those three areas.
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.
If there are, then it's a great lesson about looking outside of one's immediate circumstance and striving towards something better.
If it's a reasonable decision for an adult to make because the trade-offs might be worth it, doesn't that mean that it would also be reasonable for a child to make the same decision for the same reason?
It's either exploitative or it isn't, the age of the developer doesn't alter the trade-offs involved.
The question should not be posed to a child, that is the law for child labour, and why we do not have children gambling on roulette wheels.