My heart breaks for you and your family, and I'm not sure I can offer any help other then that my Mom died from Breast Cancer when I was six and from my point of view as a child I wish everyone around me was very upfront. Don't assume they don't understand, or won't understand because they are actually aware of way more then you may think, and it could really be scaring them. Make sure to talk to them about it often (Don't harp on it, take their cues, but always let them know you are there to answer anything they are wondering). Ask them if they are scared, what they are scared of, and let them know your feelings too. (Like if they are scared to loose their Mom let them know you are scared too, but you guys will have each other to get through it).
If you are stressed or frustrated I think it is OK to says, you know, I'm sorry, I'm just missing Mommy right now.
Keep her alive around you. Talk about her, laugh and remember her.
I have a 5 and 7 year old right now and my Dad just died from Cancer. I know it is nowhere near the same situation, but I really tried to talk to them through it, and not pretend they didn't realize what was going on. (I also didn't push it either).
Also make sure their teachers and school are always arware of what is/has happened. I remember being a kid and having other kids tease me because my mom died. Now I know it was just because it was something that was different about me, but I don't think teachers ever really knew.
I hope some of this helps. My biggest fear is leaving my children and having them face what I had to.