It is not only "FANS" who tweet these things to her. Once you are on a public platform, it should be expected that anything can come her way. If she just wants to be left alone to grow up without being bothered by fans or others, either she should be off twitter or have her twitter account private. You cannot control other people, only yourself. If you would like your circumstances to change, you have to start with yourself.
If she is being tweeted inappropriate things by sickos who recognize that she has a twitter account, then she has to accept the responsibility for that. She and her guardian. By opening up a twitter account that is public, you are saying you are willing to accept all that goes along with that, good and bad. To solve this problem is very simple, yet she is not willing to take the steps to do so and no one seems inclined to make her do it. Therefore, what does anyone expect?
I do think she should be allowed to just grow up and live as a child. However, if public accounts are making this not possible, then the public are not the ones to blame IMO.