Sure!
First, MCCC is carried on CurseForge, or googling "deaderpool" will bring up the website. It installs like any other mod.
MCCC does so many different things, I can't type up a whole manual for it here. Not that I'm knowledgeable enough to do so anyway. But a quick and dirty how-to for the copy-paste appearance function:
Click the sim, then choose MC Command Center. MC CAS, Copy Paste, Paste Tray Sim, Any Saved Sim. At this point, an alphabetical list of all of your saved households comes up. Click the one where you saved the newly aged-up child. Under that, all sims in that household that are suitable to paste over that sim come up (I know you can't paste the opposite gender over anyone, for example). Next, one last menu. Here you can choose whether or not to overwrite their name, traits, etc. We're just trying to roll back this kid's appearance, so de-select all these options. "Paste Sim Now" and it's done.
A cool thing about this feature, is that you don't have to control the sim getting pasted over. So for example, you don't care for the Goth family remake. The "Classic Goth Family" has been uploaded to the gallery. So if you download them to your library, you could put them back the way they were without taking over the household.