Say I have a parent scene and one of the child scenes is a kinematic body 2d (Or other 2d nodes with collision shapes)
How can I check if a single point (relative to the parent) is inside the child scene?
If the collision shape has a RectangleShape2D, you could use its extents property to determine a bounding rectangle.
The method Rect2.has_point() is convenient for determining if a point is inside a rectangle.
That's just an outline. The details will need to be worked out.
I think you can use Geometry.is_point_in_polygon()