I feel your pain, my family is only 4 people but I'm already strategizing and planning my packing list for our 13 day trip in January with a hotel stay, then 7 night cruise, then 4 nights at a hotel. I like to use packing cubes and then just split things up between the suitcases. We'll probably take 3. Two for clothes and one for all the other stuff - shoes, snorkel equipment, FE stuff, etc. Everyone has their own cube color. Also, if you have younger girls (I have an almost 3 year old) then dresses are awesome to pack because they're a full outfit. I pack of lot of casual cotton dresses from Old Navy or Carters. I pack mix and match stuff for my son - shorts are khaki, navy, or gray and all of his shirts match with them. I do roll things into outfits and put them in the cubes, because that way he can just pull out a roll and put it on. But we usually need to do laundry on our longer trips. Depending on the ages of your kids they may get more re-wear out of things than mine do. My kids are almost 3 and 5 and pretty much only get one use out of each outfit, especially once they eat dinner in it. But I'd imagine older kids can rewear stuff, especially things like shorts.
Oh and my other tip is to segregate out things if you can. For example we have a single night hotel stay before the cruise. So I'm going to put one outfit, PJs, and swim stuff together for that one night for me and the two kids in one cube. DH is on his own, lol. Then everything else in our suitcases can stay packed for the cruise.