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
Neuroblastoma