I consider myself a fairly knowledgable and intelligent educator.  In kids lingo, "he's smart."   However I've probably said words like "I don't know" thousands of times to the kids.  I've also told them how I don't know everything, that sometimes I make mistakes, and when I do goof in front of the class and get something wrong (sometimes it's on purpose, sometimes it's a goof) I just admit it (of course with my satirical self deprecation)

Not once did it ever hurt my standing with the kids to say, "I don't know." In fact they respect that.  Moreover, I will make it a point to find out and provide the answer next class.  They respect that even more.

When you don't know something, the kids will detect that you don't know something.  Again, never lie to them.  Each child in that room knows plenty of things that you don't as a teacher.  Each child in that room is smarter than you in some way, shape or form. 

When you accept that as your comfort level, it's real easy not knowing something.  You simply feel free to say so.