ChildComponent.cmp

<aura:component extends="c:AbstractBase" controller="TestApexController">
    <aura:attribute name="message" type="String" default=""/>
    <aura:attribute name="serverMessage" type="String" default="" description="server message assigns to this"/>
    <aura:attribute name="showError" type="Boolean" default="false" />
    <aura:attribute name="showServerError" type="Boolean" default="false" />
    <ui:inputText label="Message" value="{!v.message}" />
    <lightning:input type="checkbox" label="Show Error" name="showErrorCheck" checked="{!v.showError}"/>
    <lightning:button variant="brand" label="Send Message" onclick="{! c.sendMessage }" />
    <br/><br/><br/>
    <aura:if isTrue="{!v.showServerError}">
    	Server Message : {!v.serverMessage}
        <aura:set attribute="else">
         	Server Message : {!v.serverMessage}
        </aura:set>
    </aura:if>
</aura:component>

ChildComponentController.js

({
	sendMessage : function(component, event, helper) {
		helper.sendMessage(component,helper);
	}
})

ChildComponentHelper.js

({
    sendMessage : function (component,helper) {
        var message = component.get("v.message");
        var showError = component.get("v.showError");
        var param = {"message":message,"showError":showError};
        helper.runServerMethod(component,"c.callApexControllerMethod",function(response){
            component.set("v.serverMessage",response.getReturnValue());
        }, function(errors) {
            component.set("v.showServerError",true);
            component.set("v.serverMessage",errors[0].message);
        },param);
        console.log(message,showError);
        
    }
})