server.js

//Some import statements are missing here. Not added.
var app = require('express')();
var server = require('http').createServer(app);
var io = require('socket.io')(server);

// Creating the socket connection.
io.sockets.on('connection', function (socket) {
    socket.on('join', function (data) {
        socket.join(data.id); 
    });
});

module.exports = function (app, express) {
    var api = express.Router();
    var gps_location = "";
  
    api.post('/device_location/:child_id', function (req, res) {
        var child_id = req.body.child_id;
        io.sockets.in(child_id).emit('new_msg', {msg: 'gps_data'}); //Sending request to registered android device.
        socket.on("gpsdata", data => {
            gps_location = data; 
        })
         //Here i want to take GPS data from android client.
        res.json({ type: "success", code: 200, data: gps_location }); //Then i can send the response to anyone who need that.
    });

    return api;
};