I did bring my own detergent. I got the wisk tablets that are in sealed bags. It kept together even when it got squished. I did laundry twice on the 7 day, and only had 2 loads to do when we got home.
In answer to the first question, I'd highly recommend 3 bathing suits. It seemed like the suits took quite a while to dry out, and if you went in during the day, then wanted to go back after dinner, and again early the next morning, a third suit was a nice thing to have if the first one wasn't completely dry.
There was a thread a while back on the kids attire for dinner, and the zip off pants were recommended. I think they would be great for boys. Also nice sneakers were okay if they don't have dress shoes. I brought dress shoes for our DS, but he said that they were too stiff and only wore them a couple of nights.
I did bring a travel steamer that I got at WalMart. I used it once to get all the wrinkles out of the nice clothes. I also used the wrinkle remover spray from Downey and it worked pretty good. We weren't super starched up, but it looked good enough!