In congenital syphillis, could symptoms go unnoticed and what would the child's life expectancy be?

Not unnoticed. The symptoms of congenital syphilis are many and typically do not go unnoticed (especially today). However, with proper treatment, people can live normal (and quite full) lives. Perhaps the most famous person with congenital syphilis was british statesman winston churchill. http://www.nlm.nih.gov/medlineplus/ency/article/001344.htm.