Lightning components Parent-Child Communication can happen in various predefined ways

1. Using aura:attribute:

      aura:attribute describes an attribute available on an app, interface, component, or event. Attribute is most commonly used communication pattern where parent component can initialize child component properties as shown below. 

1. First we define attribute using aura:attribute

ChildComponent.cmp

<aura:component>
	<aura:attribute name="firstName" type="String"/>
        <aura:attribute name="lastName" type="String"/>
</aura:component>

2. We can set values while adding component in any parent component.

Parent Component.cmp

<aura:component>
	<c:ChildComponent firstName="John" lastName="Doe"/>
</aura:component>

2. Using Aura:Method

Use to define a method as part of a component’s API. This enables you to directly call a method in a component’s client-side controller instead of firing and handling a component event. Using simplifies the code needed for a parent component to call a method on a child component that it contains.

Above example can be modified in following way:

ChildComponent.cmp

<aura:component>
	<aura:method name="sampleMethod" action="{!c.doAction}" 
  description="Sample method with parameters"> 
    <aura:attribute name="param1" type="String" default="parameter 1"/> 
    <aura:attribute name="param2" type="Object" /> 
</aura:method>
</aura:component>

ChildComponent.js

({
    doAction : function(cmp, event) {
        var params = event.getParam('arguments');
        if (params) {
            var param1 = params.param1;
            // add your code here
        }
    }
})
</aura:component>

ParentComponent.cmp

<aura:component>
	<c:ChildComponent aura:id="compB"/>
        <lightning:button label="Call child method" onclick="{! c.onAction}" />

</aura:component>

ParentComponent.cmp

({
    onAction : function(component, event, helper) {
        var objCompB = component.find('compB');
        objCompB.sampleMethod("Param1", "Param2");
    }
})