En ocasiones necesitamos resolver datos de HIJO a PADRE en REACT, aqui la solucion

Parent.js

import React, { Component } from 'react';
import { Text, View } from 'react-native';
import Child from './Child.js';

export default class Parent extends Component {
    constructor() {
        super();
        this.state = {
            result: 0
        };
    }

    getResponse(result){
        this.setState({result});
    }

    render(){
        return (
            <View>
                <Text>{this.state.result}</Text>
                <Child num={2} callback={this.getResponse.bind(this)}>
            </View>
        );
    }
}

Child.js

import React, { Component } from 'react';
import { Button } from 'react-native';

export default class Child extends Component {
    calc(){
        this.props.callback(this.props.num * 2);
    }

    render(){
        return (<Button onPress={() => this.calc()} title="Calc" />)
    }
}