That’s a hard one and honestly I don’t have a clue what I’d do. I do have a friend who was around 20 when she unexpectedly became pregnant. They had no clue what they were doing until right around the delivery date. He did move in with her and her mom, they wound up getting married, having 2 more kids and are happily wed 15 years later.
I think if nothing else you encourage them to behave appropriately in terms of focusing their attention on what’s best for the child. As far as some of your other questions, it becomes tough with younger children, supporting the situation while still encouraging the other kids to maintain whatever moral standards you do have.
This is such a situational answer because much depends on the kids, their maturity level, were they in a healthy loving relationship, are the other parents of the pregnant kids cooperative? In the example above, the young man’s mother went ballistic and vowed to have nothing to do with any of it but closer to the birth and after, they supported the new parents and have been great grandparents. Situation would have been much different had they continued to maintain their initial stance.