As their immediate descendents, it is a child's right to put whatever they like in their parent's obituary. No one had the physical, social, or genetic connection they had and so no other person could possibly have known the person as well. However, I do not think it is a good idea for a child to express those negative aspects of a parent's personality. The person is dead, nothing said or written will ever change what they did, nor will it effect that person. However, writing about abuses in an obit will potentially effect the child because they have attacked the dead and their parents. For that reason, a child should be allowed to vent about abuses in an obit but should not be encouraged to.
Expressing negative feelings in an abusive parent's obituary gives a child closure without harming anyone. The abusive parent is gone, so the words can't hurt. As the closest relative to the deceased, it is up to the surviving children to write the obituary they want for their dead parent. Sometimes, this is the only way a child can discuss what happened and how they feel about it.
When people die we have a tendency to only want to remember the good things about said person. If that person was a horrible human being and mistreated people, especially their own children, then that should be made known. Just because you die does not give you a clean slate. These people went through Hell and they should be able to express their grievances without anybody condemning them for telling the truth.