Answer
It takes $0.153~s$ for the wave pulse to travel from one child to the other.
Work Step by Step
We can find the wave speed along the string:
$v = \sqrt{\frac{F}{\mu}}$
$v = \sqrt{\frac{8.0~N}{1.3\times 10^{-3}~kg/m}}$
$v = 78.45~m/s$
We can find the time for the wave pulse to travel a distance of 12 meters:
$t = \frac{d}{v} = \frac{12~m}{78.45~m/s} = 0.153~s$
It takes $0.153~s$ for the wave pulse to travel from one child to the other.
