We stayed at the Sofitel Metropole with our then 10 1/2 months old baby. The baby cot filled the room, but there was still room to move. We went in January, and it was too cold to use their outdoor pool. Our baby surprisingly really enjoyed the water puppet show, which was just a few minutes walk from the hotel. Vietnamese LOVE babies, especially the Caucasian ones, so everything was easy. We got a private one-day tour in a van to Halong Bay through the Concierge and everything was perfect. We carried our own car seat, so the baby slept fine during the car ride. The cyclo ride from the hotel to the old town was fun too.
Have fun.