not supposed to - tidak seharusnya?
"He already ate his birthday cake, although he knew he was not supposed to (do that)."
Dia sudah makan kue ulang tahun, walaupun dia tahu tidak seharusnya?
Or maybe should I say it like this:
Dia sudah makan kue ulang tahun, walaupun dia tahu tidak boleh begitu?
Sorry for my bad Indonesian!