I do not think that you can expect admissions results as good as “two top 20 schools” even for the high school valedictorian. Your older child’s results sound like they were very good.
As one example, a while ago I checked on-line and it seems that there are about 35,000 high schools in the USA, including both public and private high schools. In a recent year MIT had something like 34,000 applicants. If you are the number 1 top student in your high school (either overall or in math and sciences), then you are pretty close to being the average applicant to MIT. Of course the acceptance rate is the single digits. I did attend and graduate from MIT a long time ago, although admissions has since of course become much more difficult over the years.
Yes I do think that for a high school valedictorian getting rejected from Harvard, MIT, Stanford, Princeton, and/or Yale are common experiences (although I do know people who have attended all of these schools – some for graduate programs rather than undergraduate).
We focused more on helping our daughters find universities which would be affordable for us and a good fit for them. They both attended schools that fit these goals. Both however got their bachelor’s degree from schools that would have qualified pretty much as safeties (at least for admissions in both cases – one was affordable only with a good merit aid but the NPC did predict exactly what was offered). Then they both went on to very good and well ranked graduate programs that were a good fit for them.
In terms of where to apply and which school to attend, my feeling is that there are a lot of very good universities. Finding a school that is a good fit is way more important than ranking. Here my views might be somewhat skewed due to getting my bachelor’s degree at MIT and having a general sense that it might not have been the best fit for me.
In terms of getting accepted to highly ranked universities, my recommendation is that each student should be genuine, and should do what is right for them. As an example, what ECs were right for me and what ECs were right for my wife and what ECs were right for my two daughters were four very different sets of things.
And at least in my experience the strongest students seem to attend a huge number of different colleges and universities for their bachelor’s degree. If they go on to a graduate program then there may be some tendency for the strongest students to concentrate in the higher ranked graduate programs. However, here ranking can depend a great deal on what you are majoring in. The best schools for mathematics versus cellular biology versus veterinary medicine versus music can be four almost completely different sets of schools (with only a small and coincidental overlap).
So overall I think that your older child did very well in university admissions, and your younger child should be focusing on fit and if applicable affordability more than anything else.
And I do think that admissions to HYPSM seems to be hard to predict and based on more than just merit. To me this does not matter much simply because there are so many other very good options.