Our house is laid out in a similar way (I imagine from the description). It's a standard terraced workman's cottage.
The kitchen (think long and thin) leads into the bathroom (the back end of the house) separated by an area where there is a door to the garden so theoretically, if that back door was open, you could literally step into the house, turn left and be in the bathroom (or turn right and walk through the kitchen and into the house).
The problem with these terraced houses is that everyone has a little back garden and behind the back garden is a passage that runs behind all the houses. I imagine (although I don't know) that the layout of this little girl's house is very similar so all you would have to do is stakeout the house a bit to get a feeling for where everyone was in the house.
He was chancing his luck that the mother didn't walk back in towards the bathroom though.