JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" id="iv" />
</br>
<input type="text" id="campo" value="10">
</br>
<select name="children-qnt" id="children-qnt">
    <option value="0">0</option>
    <option selected value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
</select>Result: <span id="resultado">10,00</span>

JavaScript

var $campo = byId('campo'),
    $select = byId('children-qnt'),
    $checkbox = byId('iv');

function byId(element) {
    return document.getElementById(element);
}

function updateResult() {

    //Ajustar valor inválido
    if ($campo.value === '') {
        $campo.value = 0;
    }
    var result = parseInt($campo.value) * parseInt($select.value);

    //Formato de real no valor
    byId('resultado').innerHTML = formatReal($checkbox.checked ? result * 2 : result);
}

function formatReal(mixed) {
    var int = parseInt(mixed.toFixed(2).toString().replace(/[^\d]+/g, ''));
    var tmp = int + '';
    tmp = tmp.replace(/([0-9]{2})$/g, ",$1");
    if (tmp.length > 6) tmp = tmp.replace(/([0-9]{3}),([0-9]{2}$)/g, ".$1,$2");

    return tmp;
}

$campo.addEventListener('keyup', updateResult);
$select.addEventListener('change', updateResult);
$checkbox.addEventListener('change', updateResult);