How much does a single entry visa to China cost, how long does it take for getting it'. Thanks.

You can get an invitation letter from the grandparents to apply their visas. But if the kids are below 18 years old, I’m afraid it will be troublesome for them to travel alone. They need to provide parents’ passports and consent letter from parents. If possible, they’d better travel with an adult.
A single entry permit costs CAD 142 per person. After you submit the application, 4 working days are needed to process it.