My DD starts school tomorrow. I really do feel for you. It would be awful for my DD, for DS and for remaining parent if anything happened to one of us and we were cheated of the moment.
Fact is though, you'll find this is just the first in a long list of things that you/Nigel have been cheated of - I distinctly remember feeling this after my mum died. Even now I still add things to the list, years later- she never met my partner, she never met her grandchildren ...
Your daughter will cope. My dad died when I was at primary school. People often ask how you manage, but the fact is you've never known any different - you only have one life, one childhood, and that's the way it is.