Who is the customer for children's toys? You might argue it's the parents because they supply the money, but I think that would be like arguing that the customer for any product is the actual customer's employer - who provides the money.

It is the child who supplies the purchase motivation and the child who your product needs to satisfy. I'd say the customer is the child.

I don't think there's anything wrong with thinking of children as customers. I do think there's something wrong with manipulating children into spending thousands of dollars on digital gems, but I think that actually generalizes - it doesn't strike me as good moral choice to manipulate anyone into wasting large amounts of money on mobile games.