Most common cause of genu valgum in children is -
Commonest type of genu valgum is idiopathic.
Amongst the given options, most common cause of genu valgum in children is rickets.
Paget's disease causes genu varum.           
OA and RA are not seen in children.
Osteoarthritis
Rickets
Pager's disease
Rheumatoid arthritis