JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout-validation/1.0.2/knockout.validation.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.8.4/moment.min.js"></script>
<input type="text" data-bind="value: ChildDateOfBirth"/>

JavaScript

ko.validation.rules['simpleDate'] = {
        validator: function (val, validate) {
            return ko.validation.utils.isEmptyVal(val) || moment(val, 'MM/DD/YYYY').isValid();
        },
        message: 'Invalid date'
    };

ko.validation.registerExtenders();

$(function() {
    ko.applyBindings(new ViewModel());
});



var ViewModel = function (model) {
    var self = this;
    
    self.ChildDateOfBirth = ko.observable().extend({ simpleDate: true });
};