I had a recent question about up-sizing designs for children to adult size. The example I was shown was one that I think would have been unflattering on an adult even though it was adorable on the child.
There are a couple of things to take into consideration when deciding if the design will work on an adult. First, children have much larger heads proportionally than adults do. Their bodies appear very narrow in comparison and the length of the body is approximately 5 head lengths at 3 years old as compared to an adult which is 8 lengths. You can see an artist's rendering here, part way down the page. It shows the head length changes at various ages.
Secondly, designs for children often include gathers or fullness that does not work on adult shapes. In the photo at the top you will notice that the child's version has an empire style line creating gathers high up on the body. The adult version does not include that detail and adds waistline definition with a belt. If the child's version had simply been up-sized we might have assumed the adult was pregnant or perhaps has an apple shaped figure. I looked at quite a few garments in google images and Ravelry by searching for mother - daughter, adult - child sizing and family categories. It seems to show that downsizing is very successful, if it already flatters an adult a design works equally as well on a child.