The issue becomes whether you and the missus want to be on a beach or something urban. The Setai is on Miami Beach on the ocean. The Mandarin is not a beach front property. There is nothing for your family to do that is "family fun" at the Mandarin or in walking distance. You will have to rent a car or take a cab to get to some activities as Miami-Dade County has horrific public transportation.

Depending on your child's age you may want to take him/her to Parrot Jungle. FWIW, for three days you should settle in for some lovely beach time, sun and warmth. You will not be getting that in your home base for a couple of months. Have fun.