Roblox takes their 75% cut out of each sale then if you ever manage to accumulate the 100,000 Robux needed to cash out ($1000 worth for what Roblox sells it for) you only get $350. They're double dipping their cut. You should really watch the video it lays out the case very well.

If it's meant to be educational it shouldn't be marketing itself as a way to make money or charging kids to promote their app. It would have better ways to surface new applications instead of only showing the top N that already have to have over a thousand users.