The Parents' echos are somehow married to your account through possibly how they were originally purchased... if you bought them by ordering from Amazon, they were already set up with your account info. In order for them to be completely separate they have to be completely removed from and forgotten by your controls, then set up separately...
You may have already done this... and I may be wrong... but this is where I would start. Complete and total disconnect Deactivate... basically reset the Echo. Complete and total re-installation and set-up.
Otherwise, there is no reason there should be someone else getting your messages.