Secondly, when your parents go to Xining, I personally suggest that June to September is the best in the Gregorian calendar every year. The temperature and humidity at this time are the most comfortable time in Xining. The rest of the time depends on your parents' health.
The third option is transportation. Personally suggest the train. Of course, the sleeper is the best. After all, you can give the elderly some time to adapt.
Fourth, prepare some medicinal materials to prevent altitude sickness, such as rhodiola, in case of emergency.