How to detect how many colliders colliding inside of a trigger in a child object?

[Kurt-Dekker]

You can get the heading (bearing) to another object using Mathf.Atan2() and the delta in position.

yAxis rotation parent object.

Line 10 is your problem. Subtracting one axis from another does not give an angle. In fact, it is mathematically meaningless. Basic trigonometry tells us that the tangent of an angle is the opposite side over the adjacent. Remember TANOA from algebra in school? The function I suggested is called Mathf.Atan2() for a reason: it takes 2 coordinates and returns you the arctangent, which is the inverse of the tangent. In other words, it maps from two orthogonal axis to an angle. This means that…

As for the aggregate “fully surrounded” part, that will be handled by some kind of count or checklist of directions.

For instance, in a grid it is easy: if you are at x,y then you just add offsets going around all 4 directions (or 8 directions) to your x,y and ask if there is something present.