My kids did not paid a cent nor did most of their friends. There are some paid a little, no more then the relatively normal amount of money. If someone 8 years old is paying a lot of money for Roblox while his friends prefer roblox because it is free, then the issue is provably solvable.

Beyond limiting infinite amount of paying by not giving the kid infinite amount of money, you can limit their time in the app or on tablet by rules like "max X hours per week".