Japanese just use kodomo as in child
良い子だね iikodane would suffice if talking to the child or telling someone about them.
You may say 'imouto.'
"tsuki komodo"
Kashikoi ko
hiko
the world child? you can say SEKAINO KODOMO(TACHI).
'black haired child' is "KuroKami No Kodomo"
You may say 'nakuko.'
tora no hotondo
chisai kitsune
'Little fire' translates toå°ã•ãªç‚Ž (chiisana honoo) in Japanese.
The term 'little sister' could be translated as 'imouto', to say 'my little sister' you could say 'watashi no imouto'. But remember, if you are going to refer to someone else's little sister, you must say 'imouto-san', 'san' being the honorific, which are a key element of the Japanese language and etiquette.