how did u get there and to the makerers WHY IS THERE A SHADOW BABY DOLL IN THE CHARACTER
To get to that secret room, jump off a box into the roof in the htube system. You can see it done here in DanTDM's video on the game (old version, but still applicable).
As for the baby, it's name is Broderick. In the final build of the game, it will have your legs; it will be the final boss. You can find it hiding in various other places. For example, those with a good eye may be able to spot it watching you in the train tubes and h-tube system. Have fun with that and I hope you look forward to the next update!