The most common leukocytoclastic vasculitis affecting children is –
Leucocytoclastic vasculitis-

It is a cutaneous acute vasculitis characterized clinically by palpable purpura especially of the legs and histologically by the presence of leucocytoclasts (Leucocytoclasts are nuclear debris of neutrophils)

Causes of leucocytoclastic vasculitis: -

Drug-induced vasculitis   
Henoch schonlein purpura    
Systemic lupus erythematosis              
Rheumatoid arthritis    
Sjogren's syndrome    
Essential mixed cryoglobulinemia              
SABE    
EBV infection               
HIV infection                                  
Chronic active hepatitis      
Ulcerative colitis    
Retro peritoneal fibrosis             
Primary biliary cirrhosis
Takayasu disease
Mucocutaneous lymph node syndrome. (Kawasaki disease)
Henoch Schonlein purpura
Polyarteritis nodosa