Will drinking softened water deprive my family of essential minerals?
Almost certainly not but, if you were deficient in minerals, such as magnesium, then any additional magnesium in hard water might be beneficial.
We get most of our essential minerals from our food not our water intake. No water, whether hard, soft or softened, can provide all or even a major portion of the minerals we need.
Having said this the World Health Organisation are currently (2008) conducting a study.