Vuejs mount the child components only after data has been loaded

Use <template v-if="childDataLoaded">,And load your child component after getting data like this

<template>
  <template v-if="childDataLoaded">
    <child-cmp :value="childdata"></child-cmp>
  </template>
</template>

<script>
  export default{
    data(){
        childDataLoaded: false,
        childdata : [];
     },
     methods:{
      fetchINitData(){
        //fetch from server then
         this.childdata = res.data.items;
         this.childDataLoaded = true;
         console.log(res.data.items) //has some values
       }
     }
   components:{
     childcmp
    },
   mounted(){
     this.fetchINitData();
     }
    }
 </script>

Here is the Nice and cleaner way to update child component.

var child = Vue.extend({
    template: "<div>Child Component : {{name}} <div v-if='loading'>Loading...</div></div>",
    props: ['name','loading']
});
var app = new Vue({
    el: "#vue-instance",
    data: {
        name: "Niklesh",
        loading: true
    },
    mounted() {
    		var vm =  this;
    		setTimeout(function() {
        	vm.name = "Raut";
          vm.loading = false;
				}, 1000);
    },
    components: {
        child
    },
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.1/vue.js"></script>
<div id="vue-instance">
    <child :name="name" :loading="loading"></child>
</div>

Just Bind key to child-cmp you get reactive output. this is simplest method of reactivity in Vue js.

<template>
  <child-cmp :key="childdata" :value="childdata"></child-cmp>
</template>

<script>
  export default{
    data(){
       childdata : [];
     },

     methods:{
      fetchINitData(){
        //fetch from server then
         this.childdata = res.data.items;
         console.log(res.data.items) //has some values
       }
     }

   components:{
     childcmp
    },

   mounted(){
     this.fetchINitData();
     }
    }
 </script>