you might want to contact the seller directly -- this is an open discussion forum for design questions generally.

Using a boxspring will block most of the bookshelf and possibly add a foot in height (depending on thickness).

How old is your daughter?

The beds dimension state it's already 15.5" high without a mattress. If your box spring is 6" and mattress is 6", it will be over 27" high to climb up. That's the height of most barstools.

Our son had a captains bed (without boxspring) from age 3 to 10. He needed a stool to climb up for the first couple of years.