It is common for feral cats to go into sewers. I don't know how they manage it, but when I lived in the city I saw them often going into the sewer. No clue how they get out. I am not sure if kittens would be in the sewers also, I would also agree, if possible try to put a board down just in case something was stuck and trying to get out.

Note - I needed to move out of the city because I was taking in too many strays :-)