Children could have died before the wills were made. If you have no sources, I would add research notes in the child's profile as well as the parents profiles, noting that no sources have been found and they were not included in the will.
The female child could have died with mother from a disease or sickness that went through the family. I have seen many families where multiple deaths occurred close together or the child is associated or confused with wrong family. Again,I would add to research notes section.