If she was not there it would make sense to concoct a story that had her passively engaged in another room outside the view of the Boys. Didn't Danielle like video games?
In his original call to 911, Damon already was saying, "well, at least we still have two lovely boys to bring up." --after having spoken of Danielle in the past tense.
Similarly, every time we turn around we have Danielle being "in the other room"=out of his sight, out of sight=another euphemism for death.
It is a little like saying she had left the building.