This is a collection of short stories I have been looking for a long time.
One story involved two children that discovered tunnels, either in their back yard or the woods that we underground and had little monsters in them. Specifically the story I remember best involved a boy needing to blend in with the monsters, or he becomes invested in some way with them and wants to be a part of them. They may be all stitched together parts. His (the kid's) grandfather had some involvement with the monsters and the kid finds a stitched together skin suit made of the monsters in his grandfathers things. He wears it and becomes one of them... That or he ends up giving his body parts to them and dies in the end.
This was in a collection of scary stories I used to check out from the public library.