child.js

var argv2 = process.argv[2] || 10000;
var dly = (argv2 > 20000) ? 2000 : argv2;

setTimeout(function() {
    process.send('作業終了。' + dly + 'ミリ秒かかりました。');
}, dly);

server.js

var cp = require('child_process');
var http = require('http');
var host = '192.168.11.20';
var port = 3333;

http.createServer(function(req, res) {
    res.writeHead(200, {'Content-Type': 'text/plain; charset=utf-8'});

    var dly = parseInt(Math.random()*10000);

    if (dly > 3000) {
        forkChilds(function(child, msg) {
            res.end('' + msg);
            child.kill();
        }, dly);
    } else {
        res.end('お待たせしません!');
    }

}).listen(port, host);

function forkChilds(callback, dly) {
    var child = cp.fork(__dirname + '/child.js', [dly]);
    console.log('forked pid: ' + child.pid);
    child.on('message', function(msg) {
        callback(child, msg);
    });
}