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DS and DD both play instruments and, whilst I enjoy listening to a piece once in a while, I have no idea about any individual notes, key signatures or other technical issues. But that's why they learn with their instrumental teachers, not me!
The strings on a violin (low to high) are G, D, A, E. E is the top space on the stave, A is two spaces below that and so on. As morningpurples said, a lot of learner books have some kind of diagram at the start so you can see the strings and the corresponding notes on the stave.
If you mean about reading music, the five lines are called a stave or staff.
If the round part of the note is in a space, use the word FACE, i.e. if the note is in the bottom space it is an F, in the next space up it would be an A etc.
If the note is on a line, use EGBDF (we use Every Good Boy Deserves Flumps to help, but you can make your own up), so if the note is positioned over the bottom line it is an E, if it is over the next line up it is a G and so on.
Is in Associated Board? If you go to their website, there are recordings to download of all the grade pieces (and accompaniments). There are CDs available as well if you are near a music shop. They don't have them for all instruments, but violin is one they do.
If it's another board, or just a book of graded pieces but not for an exam, then it might be harder - there are various online sites that would teach you how to read music, or virtual keyboards that you can play if you can work out what the notes on the staff are to hear the intervals (you'd have to know the rhythm too to actually play the piece though).
Or more realistically, let his teacher show him and you can just listen and see if it sounds vaguely tune-like!! If anything is really wrong, you might just be able to hear it.
Most beginner violin books have pictures of the four strings and the first few fingers on each one, and which notes they are, which won't help you know what it should sound like, but will show approx where the fingers will go at least, and then you might be able to hear if it sounds in tune.
You tube also has a lot of graded violin pieces if it's some of the main exam board pieces.