Ya the title seems a little strange. I read it and think, "Dude you have some strange Growth on the back of your neck. Check it out. - WAAAAA! It looks like a BABY!" or some strange thing like that.
I don't think Pinkie forgot the diapers. I think after the first time she had one in her mouth. She said, "F... THIS! If it's not cake, then it does not go in this mouth. Time to brake out the book Twi gave me. Here it is, *Everypony Poops*. Ok, kids. Time to grow up and worship the porcelain Celestia."
Those. Were. Phases! They would've past by now and would need to learn all over again! *Nice mode activated: !BOOP!* Still, it is possible for them to potty train themselves! It might have even happened in real life as well. (Bloody goddamn slobbering HUMAN babies... hate them... animals on the other hand... ADORABRU!)
My second middle sister's kid is nearly a year old and STILL isn't potty trained. She claims 'She's too young to potty train! I'll have her potty trained BEFORE her official first birthday!' Well her first birthday was on the first day of winter and my niece Trinity is STILL in diapers her mother's excuse 'I've been too busy to potty train her! I simply don't have much time to stand and watch her on the toilet!' My mom got pissed and gave her a kids potty saying 'Use this you moron! Put this somewhere private and let HER choose when! You don't have to put her on the toilet nor do you have to watch her! Do I have to do EVERYTHING for you and that louse of a husband of yours?!'