Causes of Hypocalcemia in a child are-
Causes of hypocalcemia 

Hypoparathyroidism
DiGeorge syndrome
PTH receptor defect (pseudohypoparathyroidism)
Magnesium deficiency
Exogenous organic phosphate excess
Vit D difecideciency
DiGeorge syndrome
Hypoparathyroidism
Magnesium deficiency
All of above