"What d'you bring your little brother for?"
"Mum's at work. I had to."
Han frowns, then his face lights up.
"Hey! Girls love little kids, don't they! Nice one. Good thinking, man."
He puts his arm round my shoulders.
"I tell you, we're on here, all sorted!"
Even with the context you've given us, this one is a little difficult.
It seems that the boys are about to try to get the attention of some girls.
I'll assume that the phrase is said with the emphasis on "on": "I tell you, we're on here, all sorted".
Here, "all sorted" means "everything's organised, we're ready to go".
In this case, to me "we're on here" means: we're doing well, things will go the way we want.
not a teacher