Formal:
* お子様 (okosama): This is the most formal and polite way to refer to a child, particularly when talking to strangers or someone of a higher social standing. It literally means "child" but can also be used for "little one".
* お孫さん (omigosan): This is the formal way to address a grandchild.
Informal:
* ちっちゃい子 (chikkaiko): This means "small child" and is a common way to refer to a young child in an informal setting.
* ちびっこ (chibikko): This means "little one" and is a cute and affectionate way to address a child.
* こ (ko): This is a very informal word for "child" and can be used when talking about a small child in a familiar setting.
Other options:
* 赤ちゃん (akachan): This means "baby" and is used for infants or very young children.
* 小さい人 (chiisai hito): This literally means "small person" and can be used to refer to a child in a humorous way.
The best way to choose the right word depends on the context and your relationship with the person you are addressing.