What extra documents are necessary to travel to China with my 16 year old grandson?

you will need to apply for a L visa as I assume you are going there on holidays. For a L visa you need:
1. A passport valid for at least 6 months and the form completed Applicant' passport with at least 6 months remaining validity and available blank pages.
2. One truly completed Visa Application Form. Please fill in every column of the application form, using "N/A" if not applicable. If the application form is not filled out truly, completely and legibly, this can cause a delay in processing or refusal of the application.
3. A recent passport-size color photo with white background stuck on the application form.
4. Documents showing the itinerary including round trip air ticket booking record and proof of hotel reservation, etc.
You can find the form here at this address：https://www.travelchinaguide.com/embassy/pdf/visa-application.pdf

I forgot, you need to submit all of these document at your nearest Chinese embassy, OR if you go through a travel agency they should do all of this for you!