I can see those comments applying to the section just north of the Hyatt Regency Miami, itself just a 15 minutes walk north of the Hyatt Central Brickell, but we haven't that experience at all in the Brickell neighbourhood.
A quick Google search mentions that Brickell is one of the safest neighbourhood in Florida and that's the feeling we got walking around there. The Brickell City Center is a very nice upscale mall. We found more interesting restaurants that we could fit in our schedule.
As for things to do with a kid, they can run around in the Underline park or take a free ride on the Metromover. It's a part of the city more oriented towards adults, but you're not far by Uber/Lyft from family attractions like the Science museum, the Children museum, the Venetian pool or Treetop Trekking.