Actually, saying you don't know, when you do, in the right context is a great teaching utility with kids.
They could ask like, why something is or does and while you know the answer, it's much better to foster their curiosity and help then learn and understand instead of just giving the answer.
My father used to say "think about it" instead of answering so he encouraged me to figure it out by myself while still being transparent and leaving the opportunity open to help me or explain it when I'm frustrated and don't find it by myself.
I think this method is better and more honest when talking to adults like in language class the teacher says "I don't speak English" while everyone knows they do but it helps to play they don't but can fall back if necessary.
There's known knowns and known unknowns. But you cannot do anything about unknown unknowns. And unknown knowns are the Akashic records but we're not going to get into that.