You don't have to make it sound like a big life event. You just have to convince USCIS of two things: (1) you intend to engage in activities that are legal for a visitor, and (2) you intend to leave the US by the end of the extension period.
For example, "My grandchild was just born and I would like to stay in the US for another 6 months to spend time with them. After that, I will return to <my home country>."