Aero spokes strength on a family bicycle I am building a new wheel for my family bike: there will be a kid seat on the back rack over the wheel. I picked up some cheap double butted flat aero spokes (not elliptical but flat).
From reading around forums these should be no different to a standard round spoke.

My question is this: is the double butted flat spoke weaker regarding the situation, not road/race but weight carrying?  

Safety is important. No I am not looking for speed I got them as they were cheap clearance (Pro-Lite Gavia Aero Spokes).
I don't think you'll have a problem. Aero spokes are no less strong than regular double butted spokes. I use them for downhill riding and whenever one breaks (due to bad handling, loose spokes or extreme impacts) it is never on the bladed flat area. They'll either break close to the nipple or close to the spoke head.

Also note that usually spokes break one by one and never all together. So even in the rare scenario that one breaks (probably because they where too loose) the wheel will be able to continue riding normally and you'll replace it and retighten as soon as you can.