someone can correct me if I'm wrong, I haven't studied Japanese for the last 4 years so my memory is rusty...but even though we're taught that 'boku' is only used by males because its more masculine way of saying "I" or "me", a lot of kids just say boku because it's just what they learn first and what they pick up from listening to adults. So boku is generally accepted as used by kids, and can also be used in familiar settings like between friends even if you're a girl.

I think you can look at it as almost slang? That's just how I sort of see it.

Again, feel free to correct me on it! haha I'm just going by memory :)