Find the maximum from a set of child nodes using a loop to track the two largest values.
Implement max_child_value(child_values), where child_values is a non-empty list of integers. Return the largest value, while tracking both the largest and second-largest values during one loop. For example, [4, 9, 2, 7] returns 9, and [-8, -3, -12] returns -3. Assume the list contains at least two values and may contain duplicates.
def max_child_value(child_values):