Swimming is the self-propulsion of a person through water, usually for recreation, sport, exercise, or survival. Locomotion is achieved through coordinated movement of the limbs, the body, or both. Humans can hold their breath underwater and undertake rudimentary locomotive swimming within weeks of birth, as a survival response. Swimming is consistently among the top public recreational activities, and in some countries, swimming lessons are a compulsory part of the educational curriculum.
Females are not allowed to swim or appear in front of non Mahram men even with full body swimsuit because such swimsuit is usually tied and shows the shape of he body. Female can swim only in front of other females as well Mahram family members like her father, real brother, son etc.
Yes a woman can swim if she is wearing the proper islamic covering. A lot of times the women swim in a place for women only.
Seeing that these boys would be at lot younger than the age of taklif and age of tamyiz, then it would be permissible to do so.
And Allah knows best.
Swimming is good and it is mentioned in Hadeeths encouraging us to teach our sons swimming among other useful skills, but it should not happen in a place where women are swimming with indecent cloth.
Swimming in such place can cause sinful act of looking at women with indecent cloth.