A somewhat formal or literary term for a small child, typically preschool age. In everyday speech, 幼児 or 小さな子供 are more common.
幼童の教育は社会の基盤である。
The education of young children is the foundation of society.
その絵本は幼童向けに作られた。
That picture book was made for little children.
Compound of 幼 (young, immature) and 童 (child). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning 'little child'.