The problem with attempting to adopt adult female clothing for children is that the breast morph is built into mesh.  You see similar situations when doing female-to-male conversions.  The most likely workaround is to use the morph brush to flatten and smooth the chest area, then save the cloth item as a new object file.  Once the OBJ file is reloaded into the scene, you can select which option will work best to get an exact fit.