Family travel Beijing to Xian

Family of four will be traveling later this month. Mom, dad, 2 small six-year-old girls (one is 120cm. Considering these options:
1. Deluxe soft sleeper -- one child and one parent in each bed.
2. Soft sleeper -- everyone has a bed (I think).
In either case, do I book seats for everyone, and is there a way to do this online (since one will be half-fare, and one will be free)?
I will appreciate any input. Thanks.