Hijacking.
I actually went through the Canadian Space Agency's astronaut application a few years ago. Made it through the initial screening and into medical certifications. Got downselected where there were about 300 candidates remaining for two positions. But I got to see a little of the process. Here are some specifics items to add to the list, some of which will be relevant if you're trying to support your child in their dreams (and have the resources to do it).
(1) 20/20 vision. They were allowing people who had laser surgery to correct to 20/20, or people who could be corrected.
(2) Be a pilot of any kind whatsoever -- even a crop duster -- but ideally military. Kids can do air cadets, or take gliding lessons or similar.
(3) Have SCUBA certifications.
(4) Have radio certifications (even HAM radio works, but more advanced is better).
(5) Speak at least one other language that is used in one of the world's space agencies -- two are better. Russian and Japanese/French would be a good choice. Russian may fall down the list of relevance soon.
(6) Have experience in an "operational environment" -- basically, are you going to go crazy cooped up in the space station with only a few people for months? For kids this might mean backcountry camping trips. For adults, this often means being deployed somewhere, in a military or similar context.
(7) Have a medical degree, preferentially, but any STEM Ph.D. will probably work. This means making sure you select courses in school that lead that direction.
I went to grad school for planetary science -- naively thinking that I could outcompete all the people dreaming of the same thing. Make sure you have a fallback plan -- something you can pivot to -- when it doesn't pan out :)
That said, all of this may become entirely irrelevant very quickly if Starship starts ferrying a hundred people to space every day.