What was the purpose of the games in BR?
In the films it was to scare the adults into compliance since in the film Japan was falling apart due to kids boycotting school and the adults not following the governments lead and such. In the books though it's just the mad government's idea of a bit of fun if I remember right.
Why did the kids not know about them?
It's never said but since it's a political act you could always pass it off that most kids don't keep up with politics. In the books though I THINK it was a secret event, It's been a while since I read it.
Where did Mitsuko and chigusa get all the changes of clothes?
They were on a school trip and were allowed to keep their things, the island was also a town that had purposely been evacuated so they could have taken some clothes from the houses.
How did Shuyas dad know he was going into the games?
He didn't? He comitted suicide two years prior due to the fact that he was struggling to cope since he was unemployed.
If all the kids were put on concious by eating the cookies, why was the adult on the bus wearing a mask?
They were knocked out by gas.