JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/knockout-validation/1.0.2/knockout.validation.min.js"></script>
<input data-bind="value: age"/>

JavaScript

var ChildModel = function(cookieAge){
    var self = this,
        age = cookieAge || "";
    self.age = ko.observable(age).extend({
        validation: [{
            validator: function (val) {
                return !ko.validation.utils.isEmptyVal(val) && parseInt(val,10) !== 0;
            },
            message: 'Please say how old this child is.'
        },{
            validator: function (val, someOtherVal) {
                if (!isNaN(val)) {
                    if (val > someOtherVal) {
                        this.message = "This child's age must be less than or equal to 29.";
                        return false;
                    }
                    return true;
                }
                
                if (val !== '<1') {
                    this.message = "(contains text) For Children less than 1, please use '<1'";
                    return false;
                }
                return true;
            },
            params: 29
        }]
    });
};

ko.applyBindings(new ChildModel());