young child; little one
Archaic noun, often used affectionately for a young girl. In modern Japanese, 幼子 (おさなご) or 幼児 (ようじ) are more common.
昔話には、いとと呼ばれる幼い女の子がよく登場する。
In old tales, a young girl called 'ito' often appears.
Archaic prefix found in classical compounds like いとけない (young/innocent) or いとどし (childish). Not productive in modern Japanese.
「いとけない」という言葉に、この接頭辞が残っている。
This prefix remains in the word 'itokenai' (innocent/young).
幼児 is the neutral, everyday term for an infant or toddler, whereas いと is an archaic literary word for a young child.
counter for people
The origin is uncertain; it may be related to classical Japanese いと (very) or a reduplication of い (child), but the exact derivation is unclear.