Did you check our School system / système éducatif thread in the Resources subforum? I'm not personally familiar with the Australian system, so I'm not sure if you'll find the answer there. Please have a look, and if not, come back and post the AGE of a child in "prep"... it will help us to establish the equivalent French level.
PS. For help typing accents on an English-language keyboard, please see our sticky. They really are important!
Oh, dear! I don't think so. I have never heard crétin used this way before. Perhaps you are thinking of la crèche, which is something like what we call day care or nursery school or preschool in the US.
Let's wait for a native speaker to help us out here. It really would be very useful to know the age of a child in "prep," at the very least... and also, perhaps, whether "prep" is part of the national school system or rather something a parent selects and pays for individually.
Yes, Jann is right, crétin does not make sense at all in this context. and it would be an embarrassing mistake if used
5 years old = grande section d'école maternelle
4 = moyenne section d'école maternelle
3 = petite section d'école maternelle
All these are the equivalent to nursery school
Primary school starts at 6 with 'classe préparatoire', usually called CP.
Hope it helps
And a warm welcome to you, ANtoinette256!