Mary didn’t know everything that would happen to Jesus. She was human, not divine, so she did not possess God’s knowledge. We know that she did know that Jesus was the Son of the Most High (remember that the angel told her that at the Annunciation), but she did not know the details of how his life would unfold. In the Gospels, when she encounters the unexpected, we see her pondering it (see Luke 2:51), which implies that she is processing the unknown.
Even though Mary didn’t know exactly how her son’s future would unfold, there is evidence that she did believe that he could do great things. The story of the wedding at Cana, in which the adult Jesus performs his first public miracle, shows that Mary trusts in what Jesus can do. Like many moms, she sensed that her child was capable of great things, and encouraged him to try (in her case, of course, she envisioned even greater things than the average mother does). Mary may not have had exact knowledge about her son’s future, but she certainly seems to have had a lot of faith.