Forked child processes does not exit - node.js

Question

I have a script downloading pdf files. These files come in batches of 5. So I thought of using fork to download 5 files simultaneously then when finished start the next 5. I made a small test file before jumping to full scale project. I found 2 problems
child process does not exit (it seem) after downloading
fork is not async. meaning I have to use some other method for waiting before sending next batch
main
const fork = require('child_process').fork;
var url_s = ["https://www.responsibilityreports.com/HostedData/ResponsibilityReportArchive/a/NASDAQ_AAON_2020.pdf", "https://www.responsibilityreports.com/HostedData/ResponsibilityReportArchive/a/NASDAQ_AAON_2019.pdf"];
var year_s = ["2020","2019"];
name ='a';
type = 'a';
const ls = fork('a.js');
ls.on('exit', (code)=>{
console.log(`child_process exited with code ${code}`);
});
ls.on('close', (code)=>{
console.log(`child_process exited with code ${code}`);
});
ls.on('message', (msg) => {
ls.send([url_s[0],name,type,year_s[0]]);
ls.send([url_s[1],name,type,year_s[1]]);
});
child
const down_path = 'test/';
const https = require('https');
const fs = require('fs');
process.on('message', async (arr) => {
console.log("CHILD: url received from parent process", arr);
url = arr[0];
name = arr[1];
type = arr[2];
year = arr[3];
await download(url,name,type,year);
});
process.send('Executed');
async function download(url,name,type,year) {
https.get(url, res => {
const stream = fs.createWriteStream(down_path + name + '_' + type + '_' + year + '.pdf');
res.pipe(stream);
stream.on('finish', () => {
console.log('done : ' + year);
stream.close();
});
});
}
I tried to kill child processes manually using process.kill() and process.exit(),
stream.on('finish', () => {
console.log('done : ' + year);
stream.close();
process.exit()
});
but it kills all the child processes, not the one I want.