You mean intro, but the end did suffer, too.I honestly think that it has nothing to do with marketing. It's all about time. They just didn't have the time or the resources to craft a coherent ending.
All you outlined would make too much sense.It would have been cool (well... you know) if you saw the mother/father and the kid run into the vent building and then it blow up when you get there but inside you just see the parent's corpse and the kid hiding in the vent, all scared and confused and then Shepard and Anderson take him with them. The kid would talk along the way, Shep could reassure him or be blunt and at the end of the Earth bit Shep would put him on a shuttle and it'd get blown up. That way, we'd have cared about him more (he'd have a name, a personality) and we'd feel responsible for his death (putting him on the shuttle that gets blown up).
But noooooooooooo we see him for all of 10 seconds in a vent being weird and then wandering around at the end before he stumbles into a shuttle and blows up.
I imagine we would have seen the kid's mom's death or her corpse somewhere. But the kid says his dad flies a big ship, so he's probably a pilot. I don't think we would be able to have seen him. However, if we had put the kid onto his impending deathbed, it would have affected Shep more than what actually happens.