The shepherds are given a sign for three reasons. First of all it is to confirm the prophecy of the angels of God, so that when they get into Bethlehem and go looking for the Saviour, they will find him in exactly the circumstances they were told.
Christ came into our world with all the vulnerability of a little child. Did that make his future uncertain or fragile? Not at all. He would be the Saviour of the world without doubt. The Father can protect the weak just as well as the strong. Every little child born into this world has a clear path ahead of them to accomplish in life the things that God has ordained.