Put a long play sound activated mini recorder inside a toy your child likes without his knowledge. Send it along with the child and listen to it when it comes back. You can find a lot of spy stuff online, and since you only need audio it should not be too expensive. Be aware of the laws in your state. If something awful comes back you may not be able to disclose it, but at least you can find some legal way to look into it. On the other hand, if it comes back with minor stuff, like appropriate discipline, you will have peace of mind.