critical velocity can be calculated any number of ways;
like threshold training it is ultimately a rather nebulous (and somewhat outdated) concept.
I've used CV and developed other approaches hinging on the velocity/duration relationship but found it more useful for middle distance athletes. it can work well for distance runners but only in
cases where the amount of quality work as been relatively low previously.
the intercorrelations are so high in distance running that it really is
difficult to say one velocity is superior to another because so many other variables need to be factored into workout selection.
what is true is that faster running is generally more useful than
slower running. I'd encourage you to pay attention to what works
and the fitter your kids get, the more likely it is they will benefit
from higher intensity training or work/rest periodization.