According to Wikipedia: "The Miami-Dade Zoological Park and Gardens, branded and commonly known as Zoo Miami, formerly known as Miami MetroZoo, is the largest and oldest zoological garden in Florida, and the only tropical zoo in the continental United States." They are open seven days a week from 9:30am-5:30pm. Zoo Miami also runs a great summer camp. In this, children have the ability to go round the zoo everyday and do fun things like feeding the giraffes and rhinos. The zoo also does lots of conservation work with endangered species eg. rhinos, elephants, and local Florida species. 

Suggestions

The park covers a large area, so bring good walking shoes - you can also travel in a Safari Cycle which holds multiple people.