Accidentaly found how. You need to get access to playground. Reach the second floor of playground using platforming section(at the middle of section, you will be on top of platforming, just turn around and you can see way to get onto balcony), at second floor you can find room with note "pit is a lie". On wall will be a PC, that you can use. Push confirm button and go back to the first floor. Just into ballon pit(it will be between Main hall door and platroming section) and you fall into backrooms. After that things are linear, WARNING: I don't know if this section just dark at all or I don't have enough light, due to normal graphic settings, but it's just dark as hell.