An adult size 8 is roughly 32-24-36, where those sizes are in inches, for bust, waist and hip respectively.
An adult size 6 is roughly 32-22-34.
Children's sizes, where only chest and waist measurements are included, are roughly as follows:
Age 10: 31½ - 28½
Age 12: 33½ - 30½
Age 14: 35½ - 32½
Age 16: 37½ - 34½
However, if one is trying to work out whether a petite adult can get into children's clothing, their height (and subsequently the
length of the clothes) would also need to be taken into account.
Info sourced from this site for wedding and bridesmaid's dresses:
http://veromia.co.uk/Size-Chart.html