Toddler
Preschooler
noun
An infancy child is a toddler. The kanji are kind of deceptive since this isn't really used for infants. It's more for small children, from toddlers on up to preschoolers.
幼児 is sort of formal, but it's commonly used to talk about small children from about one to six years old, or about that specific period of early childhood.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
そのゲームは、幼児にはまだ早いんじゃないかな？
I think that game is still too advanced for preschoolers.
幼児期は特別な時期で、幼児教育ってすごく大事なんです。
Early childhood is a special time, and early childhood education is very important.
幼児が幼児語しゃべってるのって、本当に可愛いよね！
It's really cute when toddlers speak toddler language!