The Drawer of Animosity

I have been in a bit of a transition the past couple of weeks. I found myself ending one lease with a two week "layover" period before my next lease begins. I live in the same town as my mom, so I simply moved into her house for the time that I would have otherwise been homeless (the new lease starts next week though, thank god).
The hardest part about my living hiatus, though, is the girl who took my place at my last house. She has been best friends with my former roommate for years, apparently, and moved down a week early to...hang out I suppose. Let the animosity begin now. I have a lot of bills and being a server doesn't exactly make me a millionaire, so I work a lot of long shifts. "Sarah," as we'll call her, started nagging at things right away. "Who left the dogs out while they went outside to smoke a cigarette?" "Who put the box of trash in the trashcan?" "Who didn't put my Sex and the City DVD back in the right place facing right side up?" And my favorite, "Do you have your stuff packed yet?"
Now I'm not intentionally a cruel person, but I have a bit of a temper, and I usually enjoy choosing the immature path in the midst of my anger. So, since the texts (yes, these were mostly through texts) began rolling in before my time in the apartment was even up, I found myself often to busy to finish packing. So, she continued to sit on the couch, watch the four New York City gals get into shenanigan after shenanigan, and send me increasingly harassing texts.
A few day after the first came and went, I finally got a truck and moved my furniture out of the family room, in which it was then currently stacked. It was hard to move around in there, and I finally had sympathy when my ex-roommate texted me and asked me to do something about it.
I was observing my seven year old niece the other day, and noticed that she does things similar to this when she feels she is being treated unfairly. Not on the same scale, of course, but she still acts in the same sense and the two twenty-something year olds did. In other words, we both acted like seven year olds.
Is this, then, the inner child that everyone deems so highly? Maybe we shouldn't be looking for the inner self that was lost with youth, but continuing to repress it so that we don't act like children in grown-up situations. Or, maybe passive aggressive jerks shouldn't be so just uptight when others stand up to them in a...fun...manner.