Have a look here http://www.eatwell.gov.uk/healthydiet/fss/.
Most of our labelling has salt listed as sodium in mg. I always check labels and if salt is one of the first three ingredients its not a good sign! I also check sodium contents per serving. Arnotts shapes are very high in salt, olives I think are good for you but would need to check packaging to see sodium content. My eldest loves olives too, in moderation and I'm sure I've heard somewhere that they are very good for you - guess its how they are prepared that matters. I think the heart foundation tick is a good indicator that a product is ok for salt, sugar and fat levels and steer clear of any chips, savoury biscuits and processed snack foods aimed at kids. Gravy, sauces (like Chicken Tonight, Satay Sauces... most of the jar premade sauces) are very high in salt and fat - damn cause they are very tasty but after reading the ingredients we dont buy them anymore. Guess I've been checking salt levels for a while now so know the main contenders.... cheese, processed meats, bacon, packet mixes, biscuits, bread. We have these things sometimes but for the most part try to keep the salty stuff away from the kids. Sliced cheese has double the salt content of a block of tasty cheese for instance - alot of checking you can do by comparing labels and opting for the lower sodium content.
Have a look here too for Choice Magazine info on food for kids http://www.choice.com.au/Reviews-and-Tests/Babies-and-Kids/Food-and-health.aspx