i thought it was any child starting prep this year gets the bonus regardless of age, they just have to be starting school this year.
I had to notify centrelink by the end of Dec with which school my dd was going to.
My dd was 5 in April last year. She received the payment at the end of last year and she received the payment today.
I only get family tax, so don't really have anything to do with centrelink and have never spoken to them about schooling. it has all just happened automatically.