I personally dont think you should tell. If you had entered the contract knowing what was happening in your body, thats a different story, but if you didn't know then you have no obligation to tell them.
I actually quit my last casual job (teaching swimming lessons) at 5w cause I thought it ws the right thing to do. I went on to lose the baby at 11w. There is no safe time, but you really dont need to do anything until you feel its the appropriate time to.
If your contract stipulates that you have such brilliant maternity leave, then thats your luck. Well donw to you!!

In my career job (I'm an engineer) I told my boss at 12w as he was going overseas for 2 weeks, and I figured it would be public knowledge when he got back. I actually didn't really start to show until 24w!!! But as soon as I told him my job got boring, I lost a lot of responsibility and I basically just hung around until I was 36.5w.
They now have a policy wherer you have to provide a letter fomr your dr if you want to work within 10w of your due date. I guess to stop pikers like me riding the system.

Anyway - I would wait til 13-14w myself, and make it very clear that while you had been TTC for a while, you genuinely did not know you were pregnant until say 8-10w?

Good luck!!