Mother Reimu: Had to let Little Ones live... Mother Reimu: Please eat up! Mother Reimu (ghost): On second thought... Reimu don't wanna die!!! <== too late though, already splitted
The really quick version (in case you don't even want to read the summary) is that the mother did eat up easy.
The writer gets into a bit of detail that the yukkuri dies in body but not in consciousness when doing eat up easy; the paste core has to be intact to keep the paste fresh and sweet, so that the paste is edible by the children. It's a cruel death, but not too bad, because when they reach and eat the core, that's it.
The marisa is reluctant to eat and scared of Reimu's gusto to eat their mother.
They bury it. The mother curses her children as she waits to suffer and rot underground, dying a slow and horrible death.
The funny thing about this story is that the yukkuri that did please eat up wasn't as "nice" or as "selfless" since a great deal of her "post eat up" experience was all about patting herself in the back for being so awesome, selfless and such a caring mother.
Doesn't help that her daughter inherited this deceptive "niceness".