A relatively small overall body type that still retains developed body features such as slightly curvy hips.
Tall women are not petite even if they are slender. Such images should be tagged skinny.
For female characters that appear fully underaged, use child if the image is rated general or sensitive, depending on the situation as long they are not sexually depicted, otherwise use loli or shota if the image is rated questionable or explicit. See howto:rate for more.