The entire basis for being emotional attached to the child is that it is a human child and it dies. That's it. That's the only reasoning Walters and co have. There's literally nothing else to it, except sad music.
That's actually the fundamental problem with almost all video game writing that attempts to emotionally connect players to events and characters: telling a player how to feel doesn't work. Demanding they feel a certain way about events, or connect to a specific character, just because you want them to, always fails. Without question. In an interactive, player driven experience the only way to develop a legitimate emotional connection between the player and events/characters is to have that connection develop naturally on the player end.
Hence why we're connected to our squad, and Shepard, and why players have vastly differing opinions on which characters they like more than others.
Except Garrus, because everyone loves Garrus.