The loss of weight is too small for a 3 meters higher floor.

If we simplify the building structure to a column, the compressive deflection ΔL/L is (almost) the same under the same boy weight, if it follows Hooke's law, regardless of the position of the load (the kid).

So, the absolute deflection ΔL is greater for greater L, (how high is the boy). And the elastic energy increases when ΔL increases for the same load.

But going back to the subject: I think that both kids spend the same energy. You could think of a 2 floors airplane, losing altitude. One person going up to the 2nd floor, could be seen as keeping the same altitude by an external observer, but his effort to climb can not be different if he does the same movement before the plane descend phase.