CTL, apparently you replied to me earlier post before reading my later one. While it’s true that the grandchildren won’t be American citizens, if both parents are U.S. citizens and they’re married, one just has to prove residency with no apparent minimum time. I don’t know how easy or hard this is, nor do I know if the State Department website accurately reflects the law.
As regards overseas adoption, instant citizenship is relatively recent. When we adopted in 1995, we had to apply for citizenship and bring the kids to a ceremony of some sort.