Hi Samantha,
my boys are 12 and 14 so may be a little older than yours. My husband and I have been very honest with telling them what is going on, however this has been done with a huge dose of humour. Not to belittle what I am going through but to try to make it easier for them to understand.
Prior to my mastectomy I was an 18DD so the boys referred to my boobs as my 'portable head pillows'! They were always a convenient pillow when they needed to have a nap when they were little! They now tell me that unfortunately one of them has been stuck with a pin and been deflated! Gotta love kids! As I had a skin sparring and nipple conserving mastectomy I do still have something there on my left side, so they really thinks it looks like a deflated baloon!
I would think that depending on age you will have to have some serious talks with them, however try to keep it simple but ultimately honest. I think that there are some great books out there that help in explaining cancer to younger children as well!
All the best
Therese