This region is known as "crotch". This is a very common word that many English speakers know, but it may be difficult to uncover in an internet search if the word is not already known. Many people also call this region "groin" but that is not anatomically correct nomenclature. The following definition fits conditions of the question precisely:
In humans, the crotch is the bottom of the pelvis (the region of the body where the legs join the torsowhere the legs join the torso) and is often considered to include the groin and genitals.
The following is a picture of a "frog crotch arrow" which illustrates the concept of "crotch":
"Crotch" is used to indicate this sort of arrangement or shape, which mirrors precisely the arrangement of human legs and the space in between.