You know how you make a player emotionally attached to your characters? By presenting their stories, evolving their personalities by actions they make, showing their imperfections so that a player can relate. It's like the basic principle of storytelling, right?
You know what not to do? Do not just state "Hello, this is your wife, and this is your child, you love them, the 5 minute intro has ended. Now care, care about them...." I don't care about the baby. I couldn't care less about it, there is completely no reason for me to care, except the plot telling me that I should. I am level 28 now, and I haven't done anything from the main quest line.
No one makes a plot revolving around a family, that you as a player experienced in a 5 minute intro. There is no emotional bond. Why Bethesda? It is basically the same deal as the father plot from Fallout 3, "Please, will you care about this father that left you completely alone after one minute introduction? It blows my mind. Who in the world thought it would be a good idea? Why?
You need to make player care for a character. There is absolutely no reason to go after that kid in Fallout 4, none, It is stupid. That is a toddler that you just saw for the first time in your life, what can possibly drive you... please share opinions. Is there anyone, who actually cared about the kid? There were thousands of options to start the story, by building up some lovable character, and then taking him away from player to make a reason to go through all that effort. But this... this is just pathetic...