Thanks for all of the suggestions and advice. We all survived and, most importantly, had a lot of fun. It ended up being colder than anticipated. The thermometer read 27 degrees the next morning when we got up.
It was a beautiful clear night, so we decided to forego the tarps so we could look at the stars. My 12 year old was in his Hennessy Scout and I put my 5 year old in my Eno DN, and I was in my WBBB.
I put my 5 year old inside a 30 degree sleeping bag and then added a fleece blanket on top. Under him he had a basic foam pad. He wore his thermal underwear, sweatshirt and stocking hat to bed (or is the correct term "he worn the stocking had to hammock"?) Then I heated up a Nalgene bottle full of nearly boiling water and dropped it into a wool sock and put the hot water bottle inside his sleeping bag with him. He basically cuddled with the Nalgene all night and kept plenty warm.
All-in-all a successful hang with my two boys. Good memories.