If you're at a table service restaurant, it may be best to just wait until you get to the restaurant. My brother's family visited LTT last week. My little niece has a dairy and sesame allergy (like, they have an epi pen for it), and my SIL is breastfeeding, so she has to follow the allergy recommendations too.
They didn't speak to the chef directly, but he prepared a separate plate of food for my SIL (and niece) to accommodate her allergies. They said it worked out just fine and were delighted with their experience.