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$children removed 
Overview 
The $children instance property has been removed from Vue 3.0 and is no longer supported.
2.x Syntax 
In 2.x, developers could access direct child components of the current instance with this.$children:
vue
<template>
  <div>
    <img alt="Vue logo" src="./assets/logo.png">
    <my-button>Change logo</my-button>
  </div>
</template>
<script>
import MyButton from './MyButton'
export default {
  components: {
    MyButton
  },
  mounted() {
    console.log(this.$children) // [VueComponent]
  }
}
</script>3.x Update 
In 3.x, the $children property is removed and no longer supported. Instead, if you need to access a child component instance, we recommend using template refs.