Which is better: to seat a young Christian woman carrying a baby she cannot stand with, or to seat an old Muslim woman?

A person standing up from their seat in transportation to let the weak and needy sit down is a good and commendable act. However, priority should be given to Muslims, so a Muslim elderly woman is to be prioritized over others. If there is severe overcrowding and there is a fear for a child's well-being, then it is permissible to let their non-Muslim mother sit. Kindness to non-Muslims is Islamically required to win them over to Islam, and demonstrating the virtues of the religion is one of the greatest means of da'wah (inviting to Islam).