Can you travel in a luxury soft sleeper (cabine for 2 people) with 2 adults an a 7 year old?

Hello, we want to take 2 different overnight trains. We are two adults and a 7 year old child. (117cm at the moment). In March and April we will be traveling in China. We wanted to buy 4 tickets in order to have a whole cabin to ourselves. We can provide 4 different passport numbers even when we are traveling with 3 people. But I read there is no guarantee you will be in the same cabin. Is that correct? Is the cabin for two people a better option? Or is it too small?