You could try setting the parents position to relative (position: relative;). Then set the child’s position to absolute. You should then be able to give the child top and bottom values (top: 0; bottom: 0;) making it stretch out the entire height of the parent. However this could cause other issues such as the child’s content overflowing out of the child container.
I think you need to rephrase the question. Unless a child is position: absolute or a float, the parent will encompass the child and therefore the child will be 100% of it’s parent’s height. I’m guessing you mean 100% of the viewport?