In the show, it is presented as if the two apartments where the kids live are in the same building. However, each apartment was shot in a different building, including the scene in the building's hallway. Even the outside shot of the building is from a different building, so they used a total of 4 buildings to present one.
The children actors were selected through an auditioning process which lasted for two weeks. They were originally selected by their school to go and audition and then later called back if the casting directors and producers thought they would be good for the role.