settings.yaml

fields:
  test_addon:
    type: test
    validate: required

test-fieldtype.js

var Child = {
  template: '<div>A custom component! that can include other childs</div>'
}

Vue.component('test-fieldtype', {

    template: `<div>
    Testing addon
    <my-component></my-component>
    </div>`,

    props: ['data', 'config', 'name'],

    data: function() {
        return {
            //
        };
    },

    components: {
        'my-component': Child
    },

    computed: {
        //
    },

    methods: {
        //
    },

    ready: function() {
        //
    }

});

test-fieldtype2.js

var Child = {
  template: '<div>A custom component! that can include other childs</div>'
}

var Child2 = {
    template: `<div>I'm a new child :)</div>`
}

Vue.component('test-fieldtype', {

    template: `<div>
    Testing addon
    <my-component></my-component>
    <my-component2></my-component2>
    </div>`,

    props: ['data', 'config', 'name'],

    data: function() {
        return {
            //
        };
    },

    components: {
        'my-component': Child,
        'my-component2': Child2
    },

    computed: {
        //
    },

    methods: {
        //
    },

    ready: function() {
        //
    }

});