Answer:
   
   For girls, pink and frilly. For boys, anything that puffs up
   enormously around their waist the second they get into the water. :-)
   
   But seriously, this would depend how the family felt in general about
   modesty. There are few exceptions for swimming, and the families who
   are most careful about this even with young girls will not allow them
   to swim in mixed groups if they are dressed "immodestly." Also, some
   men and boys will not go out with their chests uncovered.
   
   The extent to which modesty is a concern depends both on the movement
   and the individual (some people are more modest than others). The more
   Orthodox a movement is, the greater the concern about modesty in
   swimming attire. Hence, it is difficult to give a broad rule. In
   general, especially revealing bathing suits, such as bikinis and
   thongs, are probably inappropriate (unless covered). Similarly, men
   should wear boxer-style trunks. Lastly, remember that it is not a good
   idea in general to start dressing very young children in ways that
   mimic sexually suggestive adult dress.
   
   A good approach to modesty is to wear a very long tee-shirt (i.e., one
   that goes to your knees) over your bathing suit. Many people do so for
   other reasons as well, such as to protect one's skin from the sun.
   There is less of a concern around the immediate family. Note that men
   and boys who are uncomfortable being bare-chested should also wear a
   T-shirt. Obviously, a dark-colored shirt should be worn so it won't go
   transparent when wet and defeat the purpose of wearing it in the first
   place...!