There is a very good public bus system thats runs up and down the main beach road in Cancun. The cost was very inexpensive, about 50 cents to get on. The center of town you can't miss, there is a large traffic circle here and a shopping mall and the Cancun Convention center. I can think of two beaches on the bay - Playa Las Perlas and Playa Langosta here the waves are calm and good swimming for kids. On the Caribbean side try Playa Ballenas, Gaviota Azul or Chac Mool, all can be reach by public beach or taxi.