While I won't even attempt to claim I remember how old the main character WAS before REINCARNATING, I must point out that this is Isekai and reincarnation. She is older than her time in this particular world.
Was she ONLY a child in Japan? Perhaps, but unless she died before the age of fifteen she would have SOME experience directly dealing with conflict.
Fighting? Physically she may be inferior, but there are other ways to directly deal with conflict, rather than watch from the sidelines. 'sides, other reincarnated or transmigrated characters have fought in battles; that she has gone for help would actually suggest a deeper and wiser understanding of her surroundings and circumstances than her physical age would suggest.
tldr: she is most likely only physically a child. Her not directly engaging in conflicts only makes sense if she is either "simply being passive" or "consciously weighing her current abilities vs the situation and deciding she can't do anything (without help)".
Post script: written on a phone, all caps used in place of italics.
