Washing or rinsing the knife in cold water will not remove this oil and can cause future problems because of the residue left on the utensil. It would seem simple enough that if you stay away from any cookies made with peanut products that you would be safe. Ingredients aren't the only enemy when it comes to cookies. This is especially important in restaurants because you don't know if you don't ask. Likewise, if the cookies have been served or stored in the same container, consider it off limits.
Many restaurants serve peanut butter cookies side by side with other non-peanut cookies.
You can't be certain that the cookies haven't touched. If ANY peanuts fall into the other compartments, whatever is in that compartment is contaminated with peanut oil. Whether it be gummy bears, chocolate chips or sprinkles, you have to use extreme caution if peanuts are served on the same buffet. Don't be afraid to ask someone for a fresh serving straight from the bag. Explain to them the danger your child faces.