I do not quite understand what kind of "toy" in question, if you mean classic gamedev, that odnoznachno the second option - all the logic on the backend.
usually the logic is not particularly difficult, and one server easily holds thousands of customers.
Rachel_Considine answered on July 8th 19 at 16:22
If we are talking about money, the server needs to check at least all the key points. Experienced game developers and say: "think of all the players cheaters a priori".