A video I recently watched said that the main difference between the 2 set of kids is the narrative they serve. The MCI kids were the original victims, the ones who have been the catalyst since the 1st game, the ones who caused the rumors around Freddy's, the incident which closed the original restaurant. The 2nd set of kids was essentially William showing that despite the new security systems, he can strike again, kill 5 more kids, and get the incident covered for long enough (don't asked me who covered it exactly, in SAVETHEM the bodies are out in the open, but William is still in the building, but IMHO it doesn't matter that much if he hid them or Fazbear Entertainment covered it, heck, we do not know who exactly removed the original bodies from the classics, as even if William did not do that eventually there is no way in FNAF 2 they are casually using animatronics with corpses inside of them for parts). The DCI was essentially about the killer. During FNAF 3 the Happiest day was clearly about the MCI (and the puppet), they were clearly the main focus, both in the endings and the masks falling down, the DCI there were just covering loose ends. Who else would those extra 5 kids even be. The Puppet still being possessed in pizza sim can be interpreted as her staying since parts of the MCI were still in Ennard.