I'm a girl, and I call my close female friends "hun" (short for "honey") sometimes. I've also heard girls call each other "babe" or "girlfriend." But if a boy called me "hun" or "babe," I would be uncomfortable and think he was trying to date me. If a boy called me "girlfriend," I would think he was gay. The safest bet is to just call anyone, girl or boy, by their name. Also, those terms you mentioned for boys - pal, kiddo, and brother - aren't necessarily suitable. "Pal" sounds old-fashioned to me. "Kiddo" can be any person (boy or girl) who is a child, or who is so much younger than you that you see them as a child. "Brother" is only common among African-Americans - the more common term in other communities would be "bro." You can also call boys "man" or "dude" to seem close (e.g. "Hey man, what's up? Long time no see." "Not much, dude, how 'bout you?") In some social circles, it's also okay to call girls "bro," "man," and "dude." None of the terms, for boys or girls, are age-specific, except for "kiddo."

There are a couple of words I've come across my close female friends calling me: "babe" and "bebe". Sometimes, to refer to a bunch of female friends, some girls say "my girlfriends", or just "the girls". However, these terms do not distinguish the age of the girls.

I don't think most guys use nicknames for girls unless they're dating- in my experience at least. I know girls have nicknames they use between each other, but since I'm a guy I don't know what's popular right now.