There are a number of expressions that can be used to say 'little one' in Italian, all centering around the word 'piccolo' which means 'small'. Adding the suffix -ino in Italian can be used to imply both smallness or affection, so piccolino would be a way to give the more matter-of-fact piccolo a softer or cuter tone in much the same way using 'little one' rather than simply 'little' would in English. Another possible word you could use is piccino which applies for much the same reason.
'Little one', as an expression, does not translate literally into Italian. This is because the word 'one' in English can also be employed as an adjective, a pronoun or a determiner- roles the literal translation of 'uno' does not enjoy.
