Being honest, like you were with your kids, is the main goal.  You have addressed it in regards of how to talk to them about sex &amp; now honesty is still the best policy in talking to them about death. 
You talked of the compassion registering in your children's eyes.  I do believe that children's faith is so much more pure than our own and reverting back to basic love &amp; faith can give us some measure of strength.  
I too will say a prayer for those girls &amp; also pray for those around them who will minister to them.  
When I was told there was a real chance I would die during my cancer treatments, I too struggled with the idea of heaven.  My friend shared with me this.  If this earth is so amazing and wonderful &amp; worth struggling to stay alive &amp; it is full of flaws &amp; sin, imagine how much more wonderful will heaven be when we are reunited with God &amp; his perfect love?  Or maybe the last few lines from ee cummings, remember,  will lift your soul a bit:

in time of all sweet things beyond
whatever mind may comprehend
remember seek (forgetting find)

and in a mystery to be (when
time from time shall set us free}
forgetting me, remember me