Ridiculously short and mundane.
I started playing the game to gauge it's appropriateness and was mildly impressed that there was a full motion video introduction. It was tough to watch, however, as dialog raced by on the upper screen with no user interaction (I could read the dialog or watch the video, but not both). There should've been an option to pause the screen and read the dialog - or a method of reading the dialog afterwards in a journal or something.
It took a bit of getting used to, but the interface was snappy enough during the core gameplay, where you navigate around the house at Twin Elms looking for small items that are randomly generated in a variety of locations based on a list that one of the characters you meet hands you. The story of the Hidden Staircase is actually based on a 1939 Nancy Drew book, though I have not read it. The DS storyline is very linear - there is no choice in dialog options or even in where to go.
After finding items from the lists of six different characters, I was playing three brief and cringe-worthy mini games that offered no challenge whatsoever, then the game was over.
I thought I was "trying out" the game, but I accidentally finished it. Because it was so great I got lost in the gameplay and storyline? Hardly. The game is finished in less than an hour. Thinking about it, all that boring FMV with the horrendous character models (couldn't they have made Nancy Drew herself look decent?) must've used up all of the space on the DS card.
Bottom Line: It's safe to let your daughter play this game, but she probably won't be too impressed with the boring gameplay and she'll finish it before you make it out of the city limits as you launch your next family vacation.