China double-stop visa

Hi,
My British family and I are travelling to London from Bangkok via Shanghai.
The original ticket was direct from Bangkok to Shanghai with a 20-hour stop-over. No problem.
However, the airline has now cancelled that flight and now told us we have to travel to Shanghai via Guangzhou. Fortunately the total time is still under 24 hours (just) BUT I am getting a lot of conflicting information about whether I will actually be allowed a free transit.
Based on my original plans I didn't organise one and I don't have time now.
Any experiene or help is much appreciated - otherwise I'm stuck with my young family in Bangkok.
Thanks