Did the second skip seem radical at first? The first time the idea of the first skip came up, it seemed like a big deal. Then it became so normal...so having DD skip K was an easy decision.
No, it never seemed radical, I think because DS started the math with this group at just 6. I think the third one seems radical for 2 reasons. First, it totally upsets our sibling order, and second, having a sophomore in high school, I'm MUCH more aware of how fast the second half of schooling goes,

. And that scares me.
Because our son turned 5 before the December cut-off, he was within the normal range for starting K @ age 4.
Technically speaking, then, our son is only accelerated by one year, so I guess starting 1st @ age 4 wasn't that radical (it just sounded that way!?). Rationalizing in that context, then, I suppose one more year still wouldn't be that radical.
He is the youngest in his class, however, and younger than most all the boys by 18-24 months.
I really, really, REALLY wish the school had the 4/5 split they envisioned -- it would have made this so much easier.