Answer:
L₁ = W×L / w
Explanation:
The scenario is shown in the image below.
At the pivot point, the torque acting on this point must be zero so that there will be easy back and forth without the heavier child.
Torque created by lighter child + Torque created by the heavier child = 0
Thus,
According to the axis system, the heavier child is left to the pivot (origin), so,
W×(- L ) + w× L₁ = 0
So,
L₁ = W×L / w