I was a bit surprised it was written out, to be honest - being that they were going for "realism."
I say "written out" only because I have to believe it was intentional - anyone who has experience with saying goodbye to loved ones knows the first thing they want the passing to know is how much they love them. It's almost second nature.
So actually I was a bit surprised Clementine didn't say it. At the same time, they literally just saw her dead parents as mindless zombies, maybe they both felt the words were taboo (thinking in character).