I heard that application for this Aug. is closed already. Not sure if you are thinking of this year.
The interview last about 30 mins. The children are taken to another room in a group to be assessed so the parents have no idea how well they did or what is asked unless your children can tell you afterwards. As far as I know, they get assessed on reading, writing and oral communication skills in English.
They are looking for students who are can listen well to instructions in English, mature/independant enough to move onto a bigger enviroment w/ much older students.