kitten
A young cat, typically under one year old. The most common meaning.
子猫が箱の中で寝ている。
A kitten is sleeping in the box.
彼女は子猫を拾って家に連れて帰った。
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.
あの子猫はもう大人だけど、体が小さい。
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.
Kana spelling is common in casual writing, children's materials, or when avoiding kanji.
Rarely used kanji form; 小 is not the standard character for this word.