Jewel have you missed the school info evenings (sorry, I am too lazy to scroll back and check whether you've said)? At ours a dentist lectured us for hours on what to put in a Brotzeit, and during Year 1 the kids had to learn about what was appropriate (beyond the dental stuff to suggest using a different type of bread each day etc. - all illustrated in colour in the HSU text book - no duplication during the same week) and were told to "teach" their parents about it.
So there are rules, and blimin' over complicated ones! :o
That said ... DD is coming to the very end of Year 3 and nobody has ever checked what is in her Brotzeit, even on school trips or Wandertag... There was a spate of odd low level bullying where a group of older kids went around threatening to tell the teacher when other kids had Brotzeit that was not allowed (juice instead of water, sweets etc.) but I think it was an empty threat :o
So there are guidelines, but not remotely enforced :o
DD (nearly 9) has a sandwich (Volkorn bread, sliced by me so fairly chunky, usually cheese or salami) and sliced apple on "short" days with 11.20 finish, and that plus a salami stick (when sandwich isn't salami) or a banana or a musli bar or a couple of big yogurt coated rice cakes on a 1pm finish with 2 breaks day. She barely eats breakfast though - maybe a yoghurt or a tiny amount of cereal or 1 slice of toast - if she had a big breakfast she might need less, but if I send her with less she comes home almost crying about hunger pains).
DS1 (6) is still at KiGa but goes for the same length days as DD as I pick them up together, and he has similar stuff to DD but about 3 times as much (he needs a bigger box), because he eats like a teen until lunch time and is then done for the day, and KiGa tell me off if he complains of hunger to them :o
I have sent chocolate or sweets sometimes, and it is fine, and not commented on, and is pretty much compulsory on Wandertag or trip days :o
Water in water bottles not juice is universal though, and more practical anyway.