A good recommendation really depends on your preferences in hotel amenities and, most importantly, location. If you don't want to leave the hotel, just about any five star OFF of Ocean Drive will suit you.

Personally, however, I think the draw of Miami is the City of Miami Beach (South Beach). The Mandarin and Four Seasons are in downtown Miami, and getting to South Beach is a hassle. South Beach itself has many fine hotels but, with a child in tow, I would avoid Ocean Drive (Victor/Tides), which is essentially the most touristy and obnoxious part of South Beach.

Good hotels start at the Ritz on Collins and go north from there, Setai, Gansevoort, Fontainebleu to name a few. The further north you go, the farther you are from the other restaurants/bars/clubs which are centered on Lincoln (an east/west road), which essentially dead ends into the Ritz.

If you want easy proximity to central South Beach, I'd go with the Ritz, which is solid, but not the very best in the chain. Another good option would be the Mondrian, on the west side of South Beach. Not walking distance to Lincoln, but a new, beautiful Morgans Hotel Group hotel which, perhaps because it is has many, many suites, draws both young hip families and singles. Note, the Mondrian is not on the beach, but has a lovely infinity pool that overlooks the bay to downtown Miami -- great at sunset.