The setai is wonderfull as is the mandarin oriental.
We had service issues at the tides but wouldn't recommend it anyway as you have a child,it's in a boisterous area.
Just a comment on the Ritz Carlton Key Biscayne
We were also down there and to be honest,though we didn't stay there we did a tour and spent 2 afternoons there.
Yes having spent one there,we went back prior to our flight home as the Tides wouldn't give us a late check out unless we paid for it..... ( )
It's huge and despite being full at the time we got great service at the restaurants and bars.
I loved their cigar havana cuba style bar inside.
That was a nice touch.