Tl;dr version: if you increase an object's dimension, the volume expansion would be greater than the surface area expansion. So while it is indeed true that you can make a skimpy outfit using same amount of material used in children's clothing (assuming you can return the material into it's original cloth form and change the dimensions as you see fit) you'd still be actually using more cloth than the ones already present in the smaller outfit.
Tl;dr version: if you increase an object's dimension, the volume expansion would be greater than the surface area expansion. So while it is indeed true that you can make a skimpy outfit using same amount of material used in children's clothing (assuming you can return the material into it's original cloth form and change the dimensions as you see fit) you'd still be actually using more cloth than the ones already present in the smaller outfit.