the policy at McDonald's may have varied from one location to another, but generally, you could not purchase a Happy Meal toy separately without buying the Happy Meal itself. Happy Meal toys are typically intended as a part of the Happy Meal package, which includes a meal for children.
However, it's worth noting that McDonald's policies and promotions can change over time, so it's best to check with your local McDonald's restaurant to see if they offer the option to purchase the Happy Meal toy separately. Some locations might have different policies or special promotions that allow for separate toy purchases.