On Chinese trains, one adult passenger could carry one child less than 1.2meters tall for free if he/she does not take an extra seat/sleeper. On domestic Chinese flights, children between 2 and 12 years old could enjoy half-price air tickets. For the train ticket price from Hangzhou to Beijing, it varies from different trains and seat/berth type. You would better check on the site by yourself. For the price of Yangtze river cruise, you could choose your preferred tour itinerary on the page and make an online booking enquiry: https://www.travelchinaguide.com/cruise/

Besides, if you’d like to take the Yangtze river cruise, you need to reach Chongqing first. From Hangzhou, you could take either train or flight there.