Not having used either program, I can't comment on the programs themselves, but fwiw, your DC would be finishing up 4th grade in the USA. I think what CTY means by the challenge level is that the instruction would be at the level of a typical US high school senior (for the course in question), and that usually, CTY-qualified sixth and seventh graders (so that would be children who were 11 or 12 by this fall, September 1st in most US school districts, with the noted standardized testing scores) would be appropriate students.

Did you try requesting the online placement test?
...pronounced like the long vowel and first letter of the alphabet...