I'm from Adelaide and bought a lightsaber in January. I was told that it shouldn't be an issue taking it as carry on in the US, however it may not be accepted on the internal Australian flight.
We flew 2 flights in the US with lightsabers in carry and no one pulled us up on it.
Coming home we decided not to risk being told to pack it in our suitcase between connecting flights and stowed them in our luggage.
Before leaving I found the measurements online and cut a pvc pipe to ensure the lightsaber blade would fit in the suitcase. I had to put them in diagonal across the case and I packed it well so there would be less chance of it flexing.
Just make sure you really play around with the lightsaber before leaving the parks. I had to return my son's lightsaber 3 times because it kept failing and mine once. A friend who we met over there also had issues with theirs and had to get loose parts replaced.