If you have the time, let others ahead of you on the security line while your kids watch (and you explain) what is going on, putting as positive a spin on it as possible (they are making sure that no one brings anything that might be dangerous on the plane, so we are all safe etc...). The last time I flew with young kids, I explained it to them ahead of time, making it sound like these are things that could cause problems if they fall on someone or spill, rather than explaining terror attacks to kids.
In general, I would recommend drilling your kids on the importance of listening to all authority figures on the trip, including TSA and flight attendants.