Answer
The magnitude of the torque about the axis due to the child is $~784~N\cdot m$
Work Step by Step
The torque is $\tau = r\times F$, where $r$ is the smallest distance between the force vector and the rotation axis, and $F$ is the perpendicular component of the force vector.
We can find the magnitude of the torque:
$\tau = r\times F$
$\tau = r\times mg$
$\tau = (2.0~m)(40.0~kg)(9.80~m/s^2)$
$\tau = 784~N\cdot m$
The magnitude of the torque about the axis due to the child is $~784~N\cdot m$
