vuetify validate child component

    <v-text-field slot="activator" :label="label" append-icon="date_range" solo
                  :value="formattedDate" readonly
                  :rules="rules">
    </v-text-field>

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

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