I've noticed something lately. Not sure exactly when it happened, or how, but I now speak in plurals more often than not. We, us, our, and so on is how I think. Even when PG and I were a conventional married mono couple, those were terms that drifted into my mindset, and now it's even more pervasive.
The main place I've noticed this trend is with the kids. You'll notice I speak plural there. Even though I have only one daughter, somehow, I have "kids". While I am not her parent, and she is not my child, S's daughter has become my "kid", and that's a little scary at times. PG and I opted to have one child largely because we didn't want to be responsible for another additional person. Now, it seems that, at least on an emotional level, I feel that I am, in some significant way.
Of all the factors that go into a combined household, the kids are among the most intensive and concerning. The rest of us are adults, and have the capacity to make our own choices and deal with our own emotions. The kids aren't quite as capable emotionally yet, and have to abide by our choices in many things. As there isn't much in the way of support or road maps for parenting in poly, we're sort of making this all up as we go, even more so than parenting in a "normal" household.
S's daughter seems to be the most outside the flow of the household. At 11 years old, she's working on her own sense of independence, which looks a lot like aping her peers, and wanting to fit in. Add a large splash of attention-seeking behavior (she comes by it honestly!), only a passing acquaintance with truthfulness at times, adolescence, academic challenges, some pretty extreme emotions, dealing with differences in her parent's homes, and it all can feel a bit overwhelming at times, even for the adults! Now, it seems that she is mine...
People who come in to see me for work ask how my kids are doing. I share the goings-on and things that we are working with, and don't think in "kid" singular any longer. How that will be in the long-term will be something to discover. I wish I had more time to prepare for adolescence, more time to grow into be a parent before supporting someone through that morass of change and self-discovery. As it is, I just hope "we" all survive intact. ;)