Most common cause of pneumonia in children is?
"Respiratory syncytial virus (RSV) is the most common cause of pediatric pneumonia"  — OsIci's Paediatrics
"Respiratory viruses are the single most important cause of community-aquired pneumonia in pediatric age group" — Jawetz

Most common cause of pediatric pneumonia → RSV
Most common bacterial cause of pediatric pneumonia → streptococcus pneumoniae
RSV
Streptococcus pneumoniae
Staphylococcus aureus
Klebsiella