Our kids were older, but a close friend of ours with a brain tumour had two girls under 6 and one 10. She was a very straight, pragmatic type and her decision was whilst being gentle and optimistic, not to hide the danger or keep the children in the dark about what was happening. She dreaded the conversations and questions, but was always surprised by how the younger girls seemed able to adapt to whatever happened. If their concerns were answered and they didn't feel they were being left out of some obvious adult loop which had everyone around them behaving oddly, they just tucked it away and after a few days accepted - or at least coped in their own way - with whatever the current reality was. Today they are as normal and happy a group of girls as you'll find.
Not sure if that helps. Certainly doesn't make the actual job of coping or telling any easier. Though I am pleased to hear that the hospital nursing team are giving you some practical support.
I've attached links to some UK sites that discuss this issue and provide some guidance, which you might not come across if you Google for information in the US.
Good luck
John L
http://www.breastcancercare.org.uk/c...hp?page_id=509
http://www.cancerhelp.org.uk/help/default.asp?page=7213
http://bmj.bmjjournals.com/cgi/conte...l/316/7135/880