Ada2012: Assertion_Policy

And if I define the pragma Assertion_Policy at Root package
specification, it will affect to the whole package hierarchy right?

No.
What your bolded text means is that (a) the pragma is placed immediately in a specification, like so:
Pragme Ada_2012;                -- Placed "immediately".
Pragma Assertion_Policy(Check); -- Also "immediately".

Package Some_Spec is --... and so on.

or (b) in a declarative part:
Procedure Outer_Scope is
   Pragma Assertion_Polucy( Ignore ); -- Declarative region.
   X : Some_Type:= Possibly_Assertion_Failing_Operation;
   Package Inner_Scope is
    -- the stuff in here would ALSO ignore assertions.
   End Inner_Scope;
   Package Body Inner_Scope is Separate;
Begin
  if X'Valid then
    Null; -- Do things on the valid value of X.
  end if; -- Because we're ignoring the invalid case.
End Outer_Scope;

So, they apply not to children, but to the spec/body/declarative-region itself.

According Ada2012 RM Assertion_Policy:

10.2/3    A pragma Assertion_Policy applies to the named assertion aspects in a specific region, and applies to all assertion expressions
specified in that region. A pragma Assertion_Policy given in a
declarative_part or immediately within a package_specification applies
from the place of the pragma to the end of the innermost enclosing
declarative region. The region for a pragma Assertion_Policy given as
a configuration pragma is the declarative region for the entire
compilation unit (or units) to which it applies.

This means that if I have a package hierarchy as per the following example:
└───Root
    ├───Child1
    ├───Child2
    │   └───GrandSon
    └───Child3

And if I define the pragma Assertion_Policy at Root package specification, it will affect to the whole package hierarchy right?
