I agree with ALMEEKER...when our sons were small ( they are now all in their 30s & have blessed us with 9 Grandchildren), I would respond to their "Why's" with..."Well,what do you think?" This was a great way to clear up any wrong or incorrect ideas they may have had about a topic and also gave me time to think of the sort of response I felt was appropriate for the situation.
Sometimes, all they really wanted was a bit of attention from me. Then,depending on their age & the "Why"...I'd change the subject, give them a proper answer or say..."Just because that's the reason/way it is...I don't know why." It is perfectly fine to say...I don't know....to a question.
The last time my 9 year old Grandgirl was here for the weekend, she asked me "Why is the sky blue?" Now, there are any number of ways to answer this...you can get scientific & explain light refraction, you can discuss God & how He made the sky blue, you can say that you're not exactly sure--but,it is a really pretty color,isn't it & would you like to draw & color a picture of it with me ? lol
I'm not sure this was helpful, but it was from the heart. And, now, I chuckle when I hear my sons try to answer the "Whys" from their little ones & look to me for help...