Do go for the 'deluxe soft sleeper' at US$100 (or there about) and use the Z19/Z20 'direct express'. IMHO timing is better too. Train travel in China can be an 'interesting' adventure what with other passenger's eating habits, opera signing and these feet! So in 'deluxe soft' you have just be the two of you, that will ensure an enjoyable trip. A small bottle of cologne / perfume will always come in handy.

If you are in Xian for only two days, then travel light if you can (if its possible to leave your heavy baggage in the hotel in PEK)

The food on the train (in restaurant car) is not bad, if you are not too picky, but get your seats for dinner early.

Do take your 11-year old to the Xian night market to see all the 'food stuff' on offer (you don't have to eat them, just look)!

Enjoy your trip!