As for sending them off to K, who cares? It's not like the kids are living in a no-learning vacuum. My son craves learning and has a goal of learning to read this year, even without "going" to preschool. We'll start K work this year b/c he wants to, but if he hadn't been asking for something official, we would have just continued on like I had planned, just learning somewhat passively. Preschool and K were created as a place to put kids while parents are at work, and also to make sure that the kids without parents who might teach them don't fall through the cracks, so when you have an involved, present parent who doesn't NEED them to be taught by someone else, those things aren't really important. (my mom sent us b/c she was working a gazillion hours a day, otherwise we probably wouldn't have been sent to Montessori, for instance)
In looking at the "things K teaches" lists, DS is already doing probably half them (of course, lists do differ, which is cracking me up), and I'm sure that the G6 are at least on the same level as DS, if not beyond (especially the girls).
In addition, if they don't go to K this year, they might not even DO K! They might just go straight to 1st, since K isn't a necessity. I didn't do official K (though I was on Montessori), just went straight to 1st and did fine academically. I am an Oct b'day and was the second youngest in my class until HS, so socially-with-other-children I was always been behind, but for school-school purposes that didn't much matter.
Since we homeschool and I don't feel like dealing with the district, I am SO thankful for the late WA age for compulsory schooling. Won't have to send in a letter to them for 3 more years.

If I were sending DS to school, I might wait to deal with the nonsense until 8, too, teaching him at home and putting him into the grade that he fits at that time. I wouldn't be just starting him off at 8 in K (and it's WEIRD to me that there seem to be people doing that???).
Last thought...my half brother repeated K b/c he was socially well behind the other kids. He has a Sept b'day (as do his full brother and sister...they were all born the same week in September, though very different years) and holding him back helped him socially, as well as taking one of the youngest and making him one of the oldest, which helped him by the time he got to HS with driving etc. His other full sibs did just fine, no needing to be held back, so it wasn't just the age, it was just his personality.
Anyway, taking out peoples' concerns about possible money-related reasons, there are plenty of reasons to either not send a kid (with present, involved, reasonably intelligent parents) to K at all, or to just delay it...