You don't?
If you think about it, when DPC is building a scene he must have a thought process as to who and what he's going to put there and why. Sometimes he'll stick a character in the background simply as a filler, or some bric-a-brac simply for décor. Other times, he'll put a specific character or object in a specific place for a specific purpose to be revealed later. I think combined with other possible evidence (the picture under the door, the short clothes, etc.), including the stool and low mirror as indications she had (has?) a kid makes sense.
For me, the only compelling argument (and granted, it's a good one) that the stepstool and low mirror are meaningless is that they are included in the Daz3D asset of the bathroom:
View attachment 1535811
However, as evidenced by the window, just because it's included in the kit does not mean DPC must use it. DPC could have put anything in that spot (a simple trash can would have worked perfectly), or he could have put nothing at all. So, either he said, "Fuck it," and left the stool and mirror in place, or he intentionally left it in place for a specific reason. My guess is the latter.
But I've been wrong before.