"can you not tell me?" --> "can't you tell me?" --> "you can tell me, right?" adding the "not" to "can" or other words like that is quite common in English when we're making a request. Rather than in a simple question, when we're just wondering something, like, "can you tell me where the train station is?", in a request when we use "not" we're asking for something to be given to us. "I do so very much wish for a little child! Can you not tell me where I can get one?" In the case of the sentence from Thumbelina, the speaker expresses a desire for a child and then asks a request, knowing that the person they're asking probably knows the answer, but can choose whether to give the information or not. "Can you not tell me where I can get [a child]?" makes a personal request from the person being asked the question, and it's polite. In a way, it's like saying, "please tell me where I can get a child." You wouldn't ask "can you not tell me where the bathroom is?" because the question "where is the bathroom?" is direct and where a bathroom is is public information rather than personal information. So many times while making a personal request we use the "not" form. It's like an appeal to pity. "You can't do this for me? Really?" A kind of pathetic feeling like that that makes it clear you're asking for a favor or making a personal request. "I need to study for the test, but I don't have any notes. Can't you lend me yours?" --> "You can lend me yours, right?" / "You can't do this one thing for me?" "I don't want to go there alone... won't you go with me?" In shorter questions, like, if you forget your phone and ask your friend, "can't I use yours?" it means you assume that you can and you're confirming. But in questions like the one from Thumbelina it's as I otherwise explained above.

It is a bit old fashioned to write "Can you not tell me", but it is correct. It would be difficult for an English learner to understand at first. You are correct, in this case you can understand it as "can you tell me". But the actual meaning is slightly more complex. I'm not sure how to explain it properly :)

Negative questions - "Will you not…?", "Can you not…?", "Did you not…" - have a different connotation from positive questions. They are not old-fashioned or strange. They serve a particular function. In your example, the negative question is used as a means of entreaty or of applying pressure. "I expect you to be able to. Can you not? Of course you can." "Can you tell me…?" would be a neutral question without pressure.