I just finished this and it really sucked me in. I read a sample chapter and just had to buy the book. It didn't bother me that she pretended the outside world didn't exist, for the reasons heyjude outlined. What bothered me was why she didn't spend any time trying to figure out the code? If I was trapped in a room like that, I'd spend each hour of each day trying different combinations, working my way up through the numbers until I got it. It wouldn't make for an exciting story, and is a lot less creepy and sad than the stuff she does try, disguised as games, but I thought all that was needed was a line of dialogue from Old Nick telling Ma that the keypad is boobytrapped if the wrong code is entered. Or something.
Apart from that, I really liked it and thought it was an interesting exploration of what someone would grow up like if they were confined like that, and how they would react to the outside world.