I don't really understand the problem so many people here have with the DLCs. I've downloaded the game, installed it, downloaded the DLC, it asked me if it should update the game, I said yes, it installed and it works.
Edit: I just saw they changed the way you install the DLCs in one of the newer versions of the game. I followed the instructions on how to install the DLC and it worked perfectly fine.
So here are (hopefully) better understandable instructions for Android:
1. Klick the download button and download the rpa file after your purchase
2. Go into a file manager on your phone, navigate to the folder Android/data/ourlife.demo.apk/files/ in your internal storage and create a folder called "game" there (without the "") if one does not already exist.
3. Move or copy/paste the rpa file to the folder you just created. The path to the DLC file should look something like this: Android/data/ourlife.demo.apk/files/game/[whichever_dlc_it_is].rpa (the name of the rpa file will be different depending on the DLC it is)
4. Fully close the Our Life: Beginnings & Always application and re-launch it.