[SPOILER] There are no page numbers in the online version of Mumu, but you will know when you are approaching this part at the end of the book--
Gerasim is told he needs to kill his dog, Mumu. After taking her for a final meal, he rows with her out onto the river in a boat, ties a brick around her neck and throws her in.
If your child is sensitive to this, you can still read up until the end (and the final paragraph or two after Mumu's death) and have a wonderful discussion in class. However, we may talk about Mumu's death.