My mother once said "truth takes time." She was talking about the big truths, the complex, complicated ones that aren't black and white and that are hard to discuss. 

While I don't have children, I do think that "simple truths" and "big truths" are tied in some ways to the age and maturity of the children involved. Sometimes we do lie to younger children because it's easier than trying to explain the big truth to them, which they may not yet be able to understand. We try to explain things in a way they will understand, even if that means little lies of half-truths. 

I think of my own parents' divorce - when I was 3 or 4 or 5, I couldn't understand the complexities of it and so my parents gave me an answer that I could understand and accept. As I got older, I realized the truth was much bigger, but it was only when I was an adult that I was really truly able to see the real "big truth." When you have complicated issues like that, you want to make sure you're protecting your kids and giving them what they can handle, when they can handle it.