Nested mapped models knockoutjs

HTML

<script src="http://ajax.aspnetcdn.com/ajax/knockout/knockout-3.0.0.js"></script>
<script src="https://rawgithub.com/SteveSanderson/knockout.mapping/master/build/output/knockout.mapping-latest.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

JavaScript

var viewModel = {};
var data = {
    id: "1",
    firstName: 'Adam',
    lastName: "Smith",
    children: [ {id: "2", name: "Bob", age: 5}, {id: "3", name: "Chris", age: 7 }]
};

var mappingOptions = {
            
            create: function (options) {
                console.log(options)
                    var self = ko.mapping.fromJS(options.data); //options.data;
                    
                    self.firstName = ko.observable(options.data.FirstName + "jaja")
                    self.MyTest = ko.observable("test");
                    //ko.mapping.fromJS(options.data);
                return self;
            }
        };


viewModel = ko.mapping.fromJS(data, mappingOptions);
ko.applyBindings(viewModel);
console.log(viewModel)
console.log(data)