how to validate child component in vue code example

Example 1: vuetify validate child component

<v-flex xs12 md3>
          <date-time-picker ref="startDate" v-model="startDate" :label="$t('start_date')"
                    :rules="[rules.required]">
          </date-time-picker>
        </v-flex>

Example 2: vuetify validate child component

props: { 
        rules: {
            type: Array,
            required: false
        }
    }