K.M.
We have brought our young kids to Mexico about a half dozen times starting when they were one. It is one of our favorite places to vacation and get away from the everlasting Minnesota winter.
Check with your insurance. For some health insurance plans, they are not valid outside the US. So, you may or may not be covered. If you are really worried, you could always purchase supplemental travel insurance. The only time we had a health emergency was when we were in the Cayman Islands. We ended up in the ER and between the pediatrician there and our MD back home, we managed to come up with a treatment plan for my 3 year old. So, I strongly suggest having a cell phone with international calling so you can contact your stateside Pediatrician if need be. Our insurance did not cover it so we just paid out of pocket. However, when we left the resort to go to the ER, we did bring our passports and were prepared to go to the airport and take the first plane to any major US city with a children's hospital if we did not feel comfortable with the care. Luckily, we did not need to.
The only other thoughts are to bring plenty of sunscreen. Get UV protection rash guard shirts for your daughter. We always bring immodium, tylenol, benedryl, bandaids, a thermometer, etc. in our carryons with us in case of any issue but have never had to use any of it.
Have fun.