Concatenating values from Child Objects to Parent Object

I have two objects:

Case and custom object [Questions__c] and with master detail relationship.

So when a child record is inserted or updated I want add values of Message_Text__c (one child record) entered and added with the value of Message_Text__c (another child record) to the Case custom field Clarification _description__c with comma seperated(field1__c,Field2__c).
If there are multiple child records I want to seperate them with ':' for example :field1__c,Field2__c:field1__c,Field2__c....
below trigger code is not working :(

trigger UpdatingCase on Questions__c(after Insert, after Update) {

List < Case > poList = new List < Case > ();
Set < Id > Po_Ids = new Set < Id > ();

for (Questions__c childObj: Trigger.new) {
    Po_Ids.add(childObj.ParentLookup__c);
}


if (Trigger.IsInsert || Trigger.IsUpdate) {
    List < Case> po = [Select Id, Clarification_Description__c, (Select field1, field2 from Child__r) from Case where Id IN: Po_Ids];
    for (Case p: po) {
        String concatenateString = '';
        for (Questions__c c: p.Child__r) {
            concatenateString += c.Message_Text__c + ',' + c.Message_Text__c + ' : ';
}
        p.Clarification _description__c = concatenateString;
        poList.add(p);
    }

This is what i want to achieve:

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