For some festivals, you may have big problem in buying the train tickets in early May. For kids, one adult can bring one kid below 11cm tall for free, but the kid should share one bed with you. If they want their own bed, need to pay the costs. It should have discounts on children from 110-150cm tall, not according to their age, however, how much discounts, it depends. I recommend a ticket booking tool for you, China Train Schedule(https://www.travelchinaguide.com/china-trains/) , you can check timestables, price available and they offer direct ticket booking service.