After grabbing the battery from the doll's house, take it back to the toy garage and put it in the slot on the back outside corner.
The elevator goes up and down automatically, so run up the ramps as high as you can go, then get on the elevator and ride it to the top.
At the top, go down the ramp, and press play when prompted on the cassette player with
. Then walk over and drop down into the cot, then on to the book for the next cutscene. Go through the newly revealed crack.
Make your way though this next section, using ramps and gaps to get over and under, you will eventually come to a big ramp that you can run up, and then jump over and down into a large area in front of the piano.
You will then get a cutscene with a book, your last crack will open up and this will unlock.