Are these more colloquial words for girl and boy or something? That's the impression I'm getting from online dictionaries, but I'm not quite sure.
They are more colloquial, but also extremely common. They are not 100% synonymous however. Nowadays, ”flicka” and ”pojke” are mostly used for young children whereas ”tjej” and ”kille” can be used both for young children but also for young adults.
”Kille” is also often used where English uses ”guy” as in ”I know a guy” or similar contexts.
”Tjej” and ”kille” are fairly new words and are being used more and more and for more and more older ages. Most young people would not refer to a 20 year-old as a ”flicka”, but rather a 7 year-old, whereas my grandmother would definitely do that. More formal language as in newspapers would avoid ”tjej” and probably opt between ”flicka” or ”(ung) kvinna”, ((young) woman).
Can kille be used in the romantic sense just as tjej is? For example "Han ar min kille" = He is my boyfriend?
So “kille” has a hard “k”, even though “i” is usually a softening vowel?
Yes. It's usually this way with loanwords. You'll hear the same in loanwords like keps for example.
Where was kille loaned from?
Wiktionary just says, 'Attested since 1924 according to Svenska Akademiens ordbok. From the dialectal kille ("kid (young of a goat); small boy") from killing (“kid”). Related to kid (killiing and kille are old diminutive forms of that word).'
Is there a little more to the story?