The idea that women and children are secluded to a certain type of "modest" clothing is tiresome and played out. Every person that has the money to dress themselves has the say in what they desire to wear. If you sexualize/slut shame a child for wearing a certain type of short clothing, then the problem does not lie in the fashion, it lies within you. Why would you possibly even imagine sexualizing a child or a woman you may not even know. Our clothes don’t define our character, but unfortunately your perv thoughts define your character.
If kids of too young of age wear crop tops too much and like them, they may continue wearing them as they grow older and it may become inappropriate. Also, in the summer I help out with VBS and some younger children wear crop tops and lift them over their heads or pull them down. I just don't think children should be introduced until they are older and know what is appropriate or not.