An excerpt from the "1810 Grimm Manuscripts" by Oliver Loo pg 285:
"For the Brothers Grimm, the Runge Märchen were the theoretical and practical ideals after which they tried to model their collection. Rölleke goes on to list several interesting quotes for Brentano, the Grimms and Arnim. He starts with a quote from Jacob: It would be good, to start in the beginning with a Runge Märchen or a few other examples… (Jan 22, 1811 to Brentano; Stieg IV, p. 161 f.). Arnim was not of the belief that the story of the Fischer was an actual Kindermärchen. Even the Juniper Tree because of a certain barbarity was not quite right to me. (End of December 1812 to the Brothers Grimm; Stieg III, p. 262). Jacob countered: that you do not hold the Märchen of the Fischer and also the Juniper as actual Kindermärchen, seems to me impossible (Jan 28, 1813; Stieg III, p. 269). But Arnim defended his criticism: Where from does it come, the one expression of Pißpott in the Fischer, on top of that quite pointless, because a hut makes it much more understandable, in a couple of boys of one of my local acquaintances it has held so, that he can not get it out of them again, all day they babble about the pißpott (Beginning of Feb 1813; Stieg III, S.273). I find that very amusing. This still happens today and can be rather frustrating to a parent."
This is one answer. Achim von Arnim, a well known novelist, does not consider that a "certain barbarity" in KHM #47 can make it a "Childrens's tale. Which barbarity he is thinking of, he does not say- it could be the killing of the boy, the killing of the step-mother, or the eating of the boy by the father. Arnim did not consider the word "Pißpot (piss-pot) to be suitable for children.
As can be seen, Jacob defended it. Since Wilhelm was mostly responsible for the later editions, it was he who made the changes to the texts. The "sanitizing" was mostly due to him. Had Jacob been more involved in the later editions, I think he would have left them alone.