How to tell the children im leaving please help!

i have been a nanny to two children since they where both born girl aged 15 and boy aged 9, have stay so long with them as I am the closest they have to a mother and I love as if they where my own, but have desided to leave the uk and return to australia as I need to start my own life as I havent been able to in this job, but I know the children are going to take the news really hard as they dont have much of a relationship with there mother.
but what I realy want your help with is how best to word it I know I have to be strong and not cry when telling them and make sure the no its not there falt, im going to say my family miss me and want me to move home but what the best way to start the conversation and what should I say?

It will be better if they can absorb the shock over time rather than just dumping it all on them at once. I wouldn't make it sound final like you're leaving and will never be coming back. Never is a difficult concept for a nine year old. Just lightly say you need to go home to your family but you will try to come back as soon as you can. In the mean-time you want them to send you emails and stay in touch. As long as they have a means of staying in contact with you, the impact will be lighter and over time they will adjust.

I suspect the change may be more difficult for you than the kids.