Must a girl who has not yet reached puberty, but who has started wearing the hijab and trousers, adhere to loose-fitting clothing, or is there no sin upon her if she waits until puberty to commit to it, given that the hijab is not currently obligatory for her?

The hijab (definition) is obligatory for an adult woman. It may also become obligatory for a girl who has not yet reached puberty if she attracts gazes, and her guardian must compel her to wear it, as he is responsible for that. If the hijab becomes obligatory for her, then it must be in accordance with the specifications of the legal hijab. However, if the hijab is not yet obligatory for her, there is no blame on her if some of the conditions for proper dress are not met, though it is preferable for her to observe them to accustom herself.