This is not how space agencies work.  This is not how mental health works. This is not how you plan a mission. Let alone a 20 year nonstop mission. This is not how human (and children's) rights work either. And you don't replace your astronauts a day before the start of mission either with an untrained unhealthy kid OR an untrained adult. Or ignore 

  a death on the team 

  and give no aftercare to the kids who witnessed it before you send them off. The lack of thinking in the worldbuilding made me give up.