Oh Minime - I know, it's horrible. I found the whole process of telling my DD a few months ago absolutely hideous.
I should imagine different children will react in different ways.
One piece of advice I will give is remember that your DDs initial reactions will change as the information sinks in. My own experience was hugely distressing - tears and hysterics from DD and I felt so awful and guilty, even though I knew we had made the right decision. She is coming round though and adjusting. It has been a gradual process needing lots of love, reassurance and consistency, both from me and her Dad, but despite the sad things happening to her family, she is still a happy and secure child who knows she is loved and that both her parents are there for her. Have your reassurances prepared, but if your DDs can't take in too much information at once, give them space and offer the reassurances over time. Be prepared to talk and answer questions over the next weeks and months and let them cry if they need to.
Having something nice planned for after you tell them sounds like a great idea. In my case I took DD to Subway (which she had been pestering me to do for a while) that evening after I told her, and said it was as a reward for doing so well in her sports day earlier that day, which lifted the mood, gave her a treat and took her mind off it for a while. It definitely helped in our case.
The fact that you are focused on your DDs reaction to the news shows that you are coming at this from the right place.
Sending lots of love and hoping that it goes as well as possible for you