Macrocytic anaemia in children is produced by all except –
Morphology of RBC 

Normal human red blood cells are biconcave discs (diskocytes) with a mean diameter of about 7.5 m.
The hemoglobin of red cells is located peripherally, leaving an area of central pallor equal to approximately 30-35% of the diameter of the cells.                                              
Cells of normal size and normal hemoglobin content (color) are termed normocytic and normochromic.
When red cells diameter is greater than 9 mm, they are referred to as macrocytes.
When red cells diameter is less than 6 mm, they are referred to as microcytes.
Variation in size of RBCs is known as anisocytosis.
Variation in shape of RBCs is known as poikilocytosis.

Causes of macrocytosis

Causes of megaloblastic anemia :

Vitamin B12 deficiency : 

Decrease intake - Inadequate diet, vegetarianism.
Impaired absorption :
Intrinsic factor deficiency
Pernicious anaemia
Gastrectomy
Malabsorption states
Diffuse intestinal disease eg Lymphoma, systemic sclerosis.
Beal resection, ilitis
Competitive parasitic uptake eg. fish tape worm infestation.
Bacterial over growth in blind loops and diverticula of bowel.
Increased requirement: pregnancy, hyperthyroidism, disseminated cancer.

Folic acid deficiency :

Decreased intake : inadequate diet, alcoholism & infancy
Impaired absorption e.g. malabsorption state
Intestinal diseases
Anticonvulsants, OCP.
Increased loss: haemodialysis
Increased requirement : Pregnancy, infancy, disseminated cancer & markedly increased haemoptysis.
Impaired use : folic acid antagonist.

Other causes of megaloblastic anemia

Thiamine deficiency                 
Pyridoxine deficiency 
Hereditary orotic aciduria  
Congenital dyserythropoietic anemia     
Di-Gueglielrno syndrome   
Hypothyroidism
Thiamine deficiency
Orotic acideueia
Copper deficiency
Liver disease