I think that this short story is from the late ‘70s. It may have been published in F&SF.
The main character is a government scientist sent to aid in the investigation of a science experiment gone sideways. The research center is gone, replaced by a meadow-size area from some other dimension/place/time. The meadow seems like an ideal place and contains the house of a small, attractive family.
Usually the investigators can only view the meadow -- they cannot enter it. They construct a small device which tosses small pebbles of dry ice towards the meadow. The pebbles most often just vanish as they cross the boundary, but, sometimes, for short intervals, the pebbles arrive in the meadow, where they slowly sublime, unnoticed by the family.
One day the family in the meadow is having a picnic. A young investigator sees the dry ice pebbles land in the meadow and, YOLO, rushes into the meadow himself. The family’s small girl sees the researcher and runs enthusiastically towards him. The researcher drops to one knee with arms wide to embrace the child. She launches herself and her sharp teeth at his throat and kills him. The rest of the family come over and they all enjoy devouring the young man.
Afterwards, the man of the family retrieves a thick book from the house. He stands near the boundary, consulting the book and making odd gestures. The meadow vanishes. Soon afterwards, the bones of the young investigator are tossed near the remaining investigators.
As the story ends, the main character suggests that the people from the other dimension will begin preying regularly on humans and speculates on how this will affect human society.