Ionic change and reverse osmosis are the two most usual methods to treat water hardness.

These processes produce a non-recommendable kind of water for children (sodium excess or fluoride deficit).

In order to prevent recurrent calcium nephrolithiasis it is very important to raise fluid intake and to drink bicarbonate-rich and soft mineral water.

Calcium bioavailability from water is similar to that from milk and induces a suppressive effect of bone resorption if child drinks calcium-rich water several times a day.

Therefore, in the case of children and people without nephrolithiasis, water with calcium concentrations between 100 and 150 mg/L may be an important dietary source of calcium, because it contributes about
8 - 16 % of the recommended daily adequate intake.

If you wish, you can to read more about this topic in the paper Acta Pediatr Esp 2002;60: 99-109(pdf) (spanish)