I always way over packed as well! I'm getting better though...
I have an 11yo boy and a 9yo girl (girls just need more space for accessories, etc.) but I definately agree with bringing more t-shirts and less shorts/pants. Just in case, I would pack or have them wear for the trip a pair of jeans or some type of long pants just in case. One sweatshirt and one pair of pj's would work well for my kids. They can always wear any of their t-shirts they might get as a souvenir as pjs or to wear. I have done wash and think it is much easier than packing more and trying to keep track of it.
Please don't laugh...but as far as underwear goes, I try to bring their "ready for the trash" underwear...they toss as they go! It saves on wash and cleans out their drawers as we do it. This way, I can pack pretty far ahead without upsetting the laundry balance.
I definately think less is more... I like my kids to dress nicely, but who will really notice if they wear the same clothes over again! (Unless the people at home who scrutinize your photos! Yes, I have a friend who notices these things!) Happy packing!