kitten
A young cat, typically under one year old. The most common meaning.
A kitten is sleeping in the box.
She picked up a kitten and brought it home.
small cat
Refers to any cat of small size, not necessarily a young kitten. Less common than the 'kitten' sense.
That small cat is already an adult, but its body is small.
I found a stray kitten in the park and took it home.
Last week, our cat gave birth to kittens.
My friend just started keeping a kitten as a pet.
The kitten is playfully batting at a ball of yarn.
Rarely used kanji form; 小 is not the standard character for this word.