Disposing of unsuccessful units. Going by the criteria from the reimu comic, a quote from the translation: "What is being sought is not "Yukkuri who will hold poo poo", but rather "Yukkuri who will not poo poo"."
These kids have just become really good at holding. The anons are watching more closely than the yukkuri know. The children that learn to hold it have to die too, otherwise the mothers will be encouraged to make children that are good and holding and hiding their crap.
Though the mother marisa never cared for them and just wants them around to help eat the food and so it won't have to give birth again.