Just to say, I was in Mexico last month with my dairy intolerant 3 year old,
and didn't have any problem really. BUT then he's not severely allergic,
which makes a huge difference. Even though I'd rather not take risks, I can
get away with them with him.

We used to buy LOTS of fruit, and cut it up. That was mostly our breakfasts.
Lunch was always out, and the place we stayed did a chicken nuggets and
french fries number for kids, which worked out fine eaten in the pool. We ate
fruit in the afternoons, and always had a store of fruit with us. Also lots
of bottled water.

The evening meal usually consisted of PLAIN fish or chicken for him, more
french fries or plain rice and/or more fruit. We bought the fruit there in
supermarkets, and it was really cheap. We also bought a HUGE box of Ritz
crackers which were a great carbo munch for all of us. We found the people in
restaurants very helpful once we explained, and it's always easier for them
really if you want something grilled plain.

Hope this is of use.
Hilda