follow up to mixed swimming, converts

Question

Dear Rabbi: You answered my questions in a vague way. Please, 1. Can we go mixed swimming if everyone were not immodestly dressed--say fully clothed? When--at what age--is a child prohibited from mixed swimming? 2. My questions regarding converts need a yes or no answer. Is the Takkana halachically valid? Did the Rabbis have authority to make such a Takkana? Thank you.