I think zip locs will work well. Even if they have to open them, it will be easy for you to repack
I think the most important thing is not to overpack any one bag. It will take you a while to get it all back in there! Check more suitcases with less stuff in each one. It seemed like they were checking people who had bigger or overstuffed carry ons. I packed one small bag with essentials, which for me were ticket wallet with ID & car reservation info, books, water, glasses, PDA & cell phone, food & tickets. The PDA & cell phone headset/charger were in a zip loc, my snacks were in another. It wouldn't have been difficult for them to search it.
I was fortunate to not be searched in my 3 flights 2 weeks ago, but I saw it happen to others. My friend (different travel route) had both her checked bag & carry on searched on both of her flights.
Maybe you could pack one small bag with change of clothes & stuff you don't need on the flight for all of you, and put that in the overhead. Then take a small tote bag for the in-flight stuff. Give the bags to different people & they shouldn't all be searched.