When I had a kid that picky, and I wanted Chinese, I called PF Changs and asked if they could make "plain" noodles (not on the menu), b/c that was one of 5 foods my kid would eat at age 2. They said no problem, we went to eat, and it was no problem (I don't even remember what they charged and I didn't care - but I tipped huge and got to eat Chinese out, which was great)...
And now, I call or email restaurants in advance all the time for my adult-onset food allergies. For some, it's to determine what they use for fats - I now have a favorite Indian place b/c they don't use ghee, they use canola oil, for all of their dishes. Most people would hate knowing this fact. I know that I can get simple dishes from them b/c they don't automatically have dairy in EVERY dish

. And I never care what they charge for my dish b/c the certainty of non-allergy is worth it (now, my kids still get a budget from me

)...
I already emailed Universal and Sea World for my trip next month. They sent me sample allergy menus at my planned restaurants and told me to request a chef or go in and chat at each restaurant (b/c I have dual allergies that tend to conflict on the allergy menus)...and no mobile ordering at any counter service. So, I'm bringing their corporate emails in case the "no, you can't go in, you have to mobile order" comes up.