When I was younger, my Mum would used to tell the people at the restaurant that I had something wrong with me and what I could and couldn't eat. Normally they were really nice and complimentary. One restaurant just piled a load of veg on a plate for me and didn't charge for it. Also another 1 gave me a paper cup of chips… for free.
I also went to pizza hut and took one of my pizza bases with me and asked if they could do a pizza on that. I called in advance, and they were so helpful!
There are lots of options really! Always tell the restaurant though and normally they will be more than happy to help!