But Christian love is something different. True love is to desire the highest and best for another. And the highest and best for another is salvation.
In this sense, love includes "tough love," reproaching the sinner, and telling the truth. So it is not only possible, but mandatory, for Christians to love everyone, even bad parents.
St Thomas answers my question well (post9). If the child reaches a place in which they are safe from a dangerous parent, it is the child’s joy and Christian obligation to witness to that parent (even if from a safe distance).