Laos Citizen appy for transit visa from San Francisco to Vientiane, Laos (Via Shanghai & Kumming)

Hello,

My sister and her son are Laos Citizen. They visited my Mom in California. They stay for 6 months. In their return trip, they will have 3 hours layover in Shanghai and 12 1/2 hours layover in Kumming before their final destination to Lao. They applied a transit one in San Francisco but Chinese consulate cannot issue them one. They were told to apply in Laos. How can I be sure they can return home without being held in China? Please advise.

Thanks in advance.