<aura:attribute name="test" type="Boolean" default="false"/>
<aura:handler name="change" value="{!v.test}" action="{!c.doInit}"/>
on parent
component.set("v.test",'newvalue');
on helper of child
<aura:attribute name="test" type="Boolean" default="false"/>
<aura:handler name="change" value="{!v.test}" action="{!c.doInit}"/>
on parent
component.set("v.test",'newvalue');
on helper of child