Speak plainly to your granddaughter, and explain that even though grandma seems to be happy about this, taking presents from people who don't really understand what they are doing is unethical and abusive. It must stop.
Another possibility is to make sure that when your wife goes out with your granddaughter, which is a nice thing for her to do and good for keeping her active, your wife takes only a small amount of cash with her - enough for lunch, say, or a coffee for the two of them - and no credit cards.
Another possibility is to make sure that when your wife goes out with your granddaughter, which is a nice thing for her to do and good for keeping her active, your wife takes only a small amount of cash with her - enough for lunch, say, or a coffee for the two of them - and no credit cards.