Stand in the ashes of a trillion dead souls and ask if honor matters. The silence is your answer. - Javik
"We built a superweapon...but it didn't work. We fought as a united galaxy.......but it wasn't enough" - Liara
The idea that as hard as you fought, it still wasnt enough is powerful. And the idea that you lay the groundwork for the victory of future civilizations is bittersweet.
The child represented Shepard's regret and sorrow, the child represented Shepard's doubts and even fears, something he would hide from the others to show no weakness. The death of the child was haunting Shepard in his dreams over the course of the war. Therefore the child is something Shepard is battling against in his mind, to overcome his doubt and so forth. To put it simply the child is always at the back of Shepard's thoughts so it is logical that he would depict the Catalyst in this way.