Depends on what you consider as the basis for determining value. I heard that if a child dies, the child is already with God and saved. If an adult, who has not been saved dies to save a child, the adult's soul is lost to God.
I attended Catholic schools as a child and it was taught that it might be better to save an adult vs. a child for the souls saving reasons. Don't know if it's doctrine or not. Sometimes hard to relate to possible Biblical Truths w/o getting tangled up in human fallibility and the arguments that go with it. I'd probably save the child.
Some would say that an unbaptized child won't enter heaven. I have a problem believing that. And if a mother chooses to risk or give up her own life to allow her child to have a life, she has made the supreme sacrifice. Therefore, she may have paid the price for her sins and be immediately welcomed by God if she dies.
However it goes, it is a harrowing situation to face and fortunately most of us are spared this decision.
There is a 1963 movie called The Cardinal which deals with this very subject. I watched it years ago and I think I will try to find it and see it again.