Xian to Beijing Z19 soft sleeper price, 3 persons the one compartment.

the fare for adult of upper berth is CNY398.5 and CNY415.5 for lower berth.
if your daughter is under 1.5 meters and shares a bunk with you, the fare for her is about CNY109.
a room has four berths, you can only take three if your daughter needs her own, you can not buy the fourth berth. China now applies real-name ticket policy that one passenger can only take one seat/berth on a train.

398.5(my husband) + 415.5(me) + 109(my daughter) = 923 CNY for 2 bunks, right? if I buy the ticket via travelchinguid, How many tickets will I have? two or tree ticket? for 2 bunks

you will have three tickets. but you need to pay for three adult tickets and the site will refund the price difference between adult ticket and child ticket after the ticket being issued.

but i suggest that you buy a soft sleeper ticket for child (75% of the full fare) for your daughter, as the berth is quite narrow and it will be uncomfortable for you and your daughter sleep on one berth.