Miami or Fort Lauderdale for 1 day with small child and no car

Posting here and in the Miami subreddit.

We are flying into Miami in the evening time next spring for a cruise at Fort Lauderdale 2 days later. We'll have 1 full day before the cruise and debating if it's best to spend that time in Miami or in Fort Lauderdale. For either cities, we'll probably be staying at a hotel close to the airport or train station. We have a young child and will only rent a car as a last resort. Which city would be better for us?