My son also has a peanut allergy, and we found all of the sit down restaurants great. We politely let our waiter know, and they have steered us in the right direction. At some, the chef has come out to guide us in the right direction as well. As for the counter service, it is mostly making sure they do not fry in peanut oil.
Luckily, her is not allergic to sesame - that would be a little more challenging. It seems that the peanut allergy warnings are getting more mainstream, but not sesame - I could see that hidden in more items.
Our biggest issue has been the baked goods - cookies, brownies, etc. There is a lot of almond/peanuts hidden in those even if they are not obvious. As stated, if it is a dessert at a table service, we have found the waiters/chefs very knowledgeable. However, there are sweets and treats everywhere, and the cart people aren't always as knowledgeable. Any hint of confusion, we move on to the next!
We have asked many times, and Butterbeer is a go!!!! I assume we would have learned differently after the first 20 or so he drank...