I don't recommend you do it, but engineer in me is trying to find technical solutions to all problems. If you can hear them presumably they can hear you on the

I don't recommend you do it, but engineer in me is trying to find technical solutions to all problems. If you can hear them presumably they can hear you on the same frequency. So you could get in contact by broadcasting on the same frequency. You can already hear them on the radio, so if they hear you on a child monitor that would be a weird-but-functional walkie talkie setup. Depending on where you are, it's possible that stress from knowing that their child monitor is unsecured is much higher than the risk of anyone within broadcast range abusing the vulnerability. Moreover, it's possible that whatever they replace the device with will have more significant problems to worry about. For example cloud vulnerable to the whole world instead of enthusiasts with radios in a small local area.