GREENWICH — A walk at the beach on a perfect springlike day nearly turned tragic Friday when a baby stroller slid down an embankment, plunging two young children — ages 13 months and 6 weeks — into the icy water at Greenwich Point. The worst was avoided when an off-duty Greenwich police officer saw the incident and dove into the water, rescuing the older child, while the mother scrambled down the embankment and pulled her infant from the water, police said. Deputy Greenwich Fire Chief Tom Nixon said firefighters were suited up and ready to hit the water when they received word that the kids were safe.