Whether they can or not should be up to them and their parents. We should not be in the business of banning things. If the kid and parent are comfortable with it, then fine. Ultimately, that is what it should come down to. You could ask this question of your own child.
No matter what the clothing item is, or what it looks like, everybody should be able to choose what they wear. It isn’t fair for people to go around telling other people, “you can’t wear this, you can’t wear that!” If your parents are okay with it, kids should wear crop tops if they want to.
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.