• one component has a ui:inputSelect and registers an event. the ui:inputSelect has a change attribute that fires a function, wherein the selected text (an ID) is put in an event and fired.
• one component that handles the event, is an aura:iteration list of a component that, on handling the event, should retrieve a list of items where a master-child lookup is the sent ID of the master object. This then displays all those child objects.
• the other component that handles the event, has three inputText fields, that are supposed to display the other fields of the master object

<aura:component controller="PrefixMasterHeaderSelectorController">
    <aura:handler name="init" value="{!this}" action="{!c.doInit}" />
    <aura:registerEvent name="updateMaster" type="c:PrefixMasterEvent" />
    <aura:attribute name="masters" type="PrefixMaster__c[]"/>
      
    <label class="slds-master-element__label" for="masterSelector">Select a master to work with:</label>
    <div class="slds-master-element__control">
        <div class="slds-select_container">
            
            <ui:inputSelect aura:id="masterID" class="slds-select" change="{!c.masterHasChanged}">
                <aura:iteration items="{!v.masters}" var="master">
                    <ui:inputSelectOption text="{!master.Id}" label="{!master.Name}" />
                    <!-- + ' (Last modified: ' + master.LastModifiedDate + ' by ' + master.LastModifiedBy.Name + ')'}"/> -->
                </aura:iteration>	
                <ui:inputSelectOption text="newMaster" label="Create a new master" />
            </ui:inputSelect>
            
        </div>
    </div>

    <c:PrefixMasterHeaderMaster />
</aura:component>

({
    doInit : function(component, event, helper) {
        // Create the action
        var action = component.get("c.getMasters");
    
        // Add callback behavior for when response is received
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (component.isValid() && state === "SUCCESS") {
                component.set("v.masters", response.getReturnValue());
            }
            else {
                console.log("Failed with state: " + state);
            }
        });
    
        // Send action off to be executed
        $A.enqueueAction(action);
    },
    
    masterHasChanged : function(component, event, helper) {
        var masterID = component.find("masterID").get("v.value");
        var masterEvent = component.getEvent("updateMaster");
        masterEvent.setParams({"master" : masterID})
        masterEvent.fire();
        
		console.debug('UPDATE MASTER = ' + masterID);
        alert('Hiya! masterEvent ' + masterEvent.getParam("master"));
    }
		
})

<aura:component controller="PrefixListChildsController">
	<aura:handler name="updateForm" event="c:PrefixFormEvent" action="{!c.loadChilds}" />
    <aura:attribute name="childs" type="PrefixChild__c[]"/>
    
	<!--   	<aura:handler name="saveChild" event="c:PrefixChildEvent" action="{!c.handleAddItem}"/> -->
    <!--	to reload the masters after adding a new child -->
    
    <div>
        <header class="slds-p-top--small">
            <h3 class="slds-text-heading--small">Childs</h3>
        </header>
        
        <div id="list" class="row">
            <aura:iteration items="{!v.childs}" var="child">
                <aura:if isTrue="{!child.Order__c % 2 == 1}">
                        <div class="slds-p-around--small" style="background-color: lightblue; border-radius: 10px">
                            <c:PrefixChild child="{!child}"/>
                        </div>
                    <aura:set attribute="else" >
                        <div class="slds-p-around--small" style="background-color: none; border-radius: 10px">
                            <c:PrefixChild child="{!child}"/>
                        </div>
                    </aura:set>
                </aura:if>
            </aura:iteration>
            {!v.testEventmessage}
            <c:PrefixNewChild />
        </div>
    </div>

</aura:component>

({
	loadChilds : function(component, event, helper) {
 	    var action = component.get("c.getChilds");
		var masterID = event.getParam("master");    
    	action.setParams({ "masterID" : masterID });
        
        console.debug('master id = ' + masterID);
        alert('load childs ' + masterID);
        component.set("v.testEventmessage", masterID);

        // Add callback behavior for when response is received
        action.setCallback(this, function(response) {
            var state = response.getState();
            if (component.isValid() && state === "SUCCESS") {
                component.set("v.childs", response.getReturnValue());
            }
            else {
                console.log("Failed with state: " + state);
            }
        });
        
        // Send action off to be executed
        $A.enqueueAction(action);
    }
})