My nephew (4) asked the question while we were watching rowing games of 2016 Rio Olympic. Ahh kids and their imagination.
The reason you row backwards is that you can use as many muscles and as much power as possible to pull the oar and push the water backwards, thus propelling the boat forwards. Facing backwards, you can use both your legs and arms. You’re pulling with your whole body instead of pushing with just your upper body. (source)
Longer explanation: read here at FrontRower.com
What interesting question your little ones have asked that made you think?