Hi Kys,
Sorry to hear you've had a bc diagnosis, but you've come to the right place, hopefull you'll meet lots of great girls with lots of great advice and friendship.
I was diagnosed at 37 with my youngest daughter 5 at the time. We had the same outlook, upfront (age appropriate) and dont hide things if you can help it. Firstly let me recommend a book to you that I SWEAR by for the kids.... http://www.bcna.org.au/store/products/my-mum-has-breast-cancer here is the link. It was PERFECT!
We made a few mistakes along the way in trying to hide things from them (mainly the 1st few days after Chemo, when I was the sickest) but mostly were up front.
I tried to continually ask them how they felt about certain things. My 15 year old kept alot to herself but my 5 year old was more up front, "Mum, can you wear your wig when you pick me up from school, cos the kids stare"....so, fair enough, I did! That sort of thing.
Let us know how you go with your lumpectomy results and good luck!
Sam x