Sharon,

You are indeed on the right  track about the other foods that affect Calcium.
 Our  diet in the US is very high in phosphorus which is probably one reason
why we need so much calcium.  We also sit on our ____s so much that
weight-bearing exercise is absent for many.  With all the soda pop that is
consumed, phosphorus can reach really large amounts in the diet.  Replacing
the missing calicum with dairy probably does little to rectify the balance
since dairy is also high in phosphorus.

You might try consulting a good renal dietitian (maybe in a dialysis center).
 They would have food lists for calcium and phosphorus.  Kidney patients
often take phosphorus binders and calcium supplements to help balance their
minerals.

Good luck.  Your children are lucky to have such a resourceful mom.
Is this an inherited problem?

Best Wishes!!
Martha