We always do laundry--especially with 2 kids and 10 day trips, we can bring half the amount of clothing, and half the number of suitcases. I also don't worry if someone needs a mid-day change, because I know I can always throw a load in the washer. And I can wash souvenier t-shirts that we purchased so we can wear them on the trip. I can't even imagine how many suitcases you'd need to bring for 4 kids (plus adults) for 8 days.
When do we do laundry? Usually a little past mid-trip, and either during afternoon break (last year) or one evening when we came home "early" (9:00 or so). My dh took dd for a nighttime swim (no sunscreen needed!), and I took my ds to the lobby to watch cartoons while the clothes were in the washer and dryer (had planned on going to the playground, but it closed at dusk).
Hints: Bring your own detergent in small ziploc bags. It's much cheaper, and you get to use your own brands, so clothes will smell the same and you won't have any allergic reactions to different detergent. I also bring a roll of quarters, since the change machine often runs out, and I hate running around trying to scrounge up change. And I find that most hotel dryers take much longer to dry clothes than mine at home, so I try to pack things that dry quicker--thinner fabrics and no denim, if possible.