$ cat m.js
const fork = require('child_process').fork
const net = require('net')

const cp = fork('./child.js')

net.createServer(c => {
  setTimeout(() => {
    cp.send({}, c)
  }, 500) // send the connection after a delay
}).listen(1234, () => {
  net.createConnection(1234, function() {
    var i = 0
    setInterval(() => {
      this.write(`${i++} `)
    }, 100) // start sending data before the connection is sent to the child
  })
})

$ cat child.js 
process.on('message', (m, c) => {
  console.log('child: got connection')
  c.pipe(process.stdout)
})