Space Needle | Visit the Needle
For the safety of all guests, weapons will not be permitted in the Space Needle. The only exception to this is law enforcement personnel in the performance of their official duties. The presenter of a ticket consents to a reasonable search for weapons before entering. Failure to comply with the above conditions will result in no admittance.
Don't know about metal detectors. But I wouldn't carry in the Space Needle and have to be asked to leave. First, it has been targeted for terrorism once before and I do not think they will take it lightly. Second, do you want to chance ruining your kids seeing the Space Needle and having to leave?
My 2 cents