Walking changes how one measures deadly supernovae.
As long as the kids move at a constant rate, special relativity comes into play.
Relativity is not an arbitrary change, but incredibly precise change.
And there is that interval that is exactly the same size.
Note: the numbers are far too big (off by 16 orders of magnitude, I just didn't want to write lots of zeros).
A brief explanation of this page.
Take measurements made by two observers written as quaternions and square them. If the first terms are the same, then the two observers are in reference frames that are moving in a steady way relative to each other. Note that the observers can be in a gravity field which is a non-inertial reference frame, but that does not cause an issue here.
The observers do not have to make sure their coordinate systems are the same. So long as both agree to compare so called natural units (a way to consistently be dimensionless), then the numbers will be the same. As long as the kids move at a constant rate, special relativity comes into play.