These are lessons learned from an extended sojourn in Colima and Guanajato, they are a little strict and best suited for a SHORT visit:
1. Squeeze lime on everything uncooked (great on mangoes!). Its a natural and effective means of laying low contamination.
2. Yes, you can eat salads, but with one caveat: make sure the restaurant states affirmatively on its menu that its salads 'take the pill', i.e. all ingredients are washed in water into which halogen pills are dissolved. The halogen kills amoeba--about the worst gastrointestinal peril that potentially afflicts everyone--indigenous and visitor alike. That's the way everyone at home handles this issue. If its an upper end restaurant, this goes without saying. If its mid-priced or lower, ask.
3. Street food is fine--just make sure it incorporates NO uncooked ingredients, such as lettuce or fruit. If you want a mango peeled by a street vendor, ask yourself in what quality water he last washed his knife. Don't miss eating at the market, in the La Fondas--everything is cooked.
4. Eat in restaurants where people dress like you. Along with salads that take the pill, this is probably your best defense.
5. I NEVER drink soft drinks at home, not ever. But I have to say that soft drinks ('refrescos') and Coke bottled in Mexico and made with sugar not corn syrup are damn good. You might like Sidral--an apple cider soda.
6. For your short visit and considering your delicate condition, you might should consider avoiding unpastuerized white 'ranch cheese'. But pasteurized dairy products like milk are fine, and yogurt especially good.
7. Fresh squeezed oj is ubiquitous--go for it. In fact juice places are everywhere.
To make a point, I think I'm going to repeat myself "Eat in restaurants where people dress like you. Along with salads that take the pill, this is probably your best defense."
I think you will find people extraordinarily friendly--Mexicans LOVE children (its said there that they cost nothing to make and a great deal of fun can be had making them), and if you are showing, well, that will be a great door opener! :-)
Chicago is my spiritual chow home