The most common malignant neoplasm of infancy is –
Neuroblastoma is the most frequently diagnosed neoplasm in infants accounting for 28-39% of neonatal malignancies' - Nelson.
Neuroblastoma 

Neuroblastoma is a malignant tumor of the autonomic nervous system.
It has following peculiarities : -
Most common abdominal cancer of childhood.
Most common cancer of infancy
Most common extra cranial solid tumor of childhood (most common solid tumor of childhood is brain tumor).
It is a neuroendocrine tumor, arising from any neural crest element of the sympathetic nervous system.
Primary tumor is localized in the localization of sympathetic nervous system.
Malignant teratoma
Neuroblastoma
Wilms' tumor
Hepatoblastoma