When I say, “Time to tidy up,” my child answers, “Okay,” but keeps playing. Does that mean they did not understand?

Answering and stopping an enjoyable activity to begin a new action are separate skills. Move closer and make the first action concrete: “Put the car in the box,” then place one item in the box yourself. Give advance notice such as, “Two more turns,” or, “When the timer rings,” and keep the amount to tidy manageable. Notice the moment your child starts—“You started putting the cars away”—rather than focusing only on the verbal reply.