Would also read the Foreign Office's advice on China.
Theft of UK passports is certainly not unknown, these children must be careful with their possessions generally.
No such thing as health and safety, if there is a hole in the road for instance you are expected to see it. Toilets can be basic outside of hotels to say the very least. I hope these children are staying in decent accommodation.
People do spit in public, this is normal behaviour.
Take biscuits / crisps so you can pig out in your room. If any of them are veggie be prepared to stick to chips and white rice.
The toilets were awful when I went 9 years ago, think 'festival' level of hygiene . I went to India once and they were just squat loos but all lovely and clean and didn't smell. Mind you the Olympic tourism may mean the public loo's are better these days.
Everyone was very nice in China. Forbidden City & Teraccotta Army are amazing.
I was only there for a week but even in that time I sussed out how to read bits of Chinese writing, it's quite logical once you recognise a few symbols.