Interestingly, capitalisation after exclamation or question marks is a relatively modern thing, if you look in Charles Dickens, for example, he doesn't capitalise after such punctuation.
In regard to your specific question, I think you have three options:
- Leave it
If you want to be as true to the originals as possible, you could leave the punctuation and not capitalise the succeeding letter. Since exclamation marks are used frequently in this book (I presume Rico and Wiseli by Johanna Spyri) Frequent capitalisation may interrupt the story.
As this is a children's book, teaching them a form of punctuation that is archaic and no longer used is a possible worry. Furthermore, it makes sense to stick to more modern rules to keep the story up to date.
- Leave it and use [sic]
This might be a slightly odd solution and I would advise against it on the whole. If you are updating the book as a whole it is simply impractical, however, if you are quoting the book, particularly if it is in more academic literature, using sic is considered the appropriate way to show that a sentence has been translated without any changes.
Personally, I would leave it (although I am a traditionalist) and make some reference to this issue in the editor's notes.