Is it a violation of HIPPA or any privacy law when a former dental office sends a letter to our minor children with their account numbers, birthday and our home telephone number printed on an address label? I really don't want the US postal service delivering private mail with our phone # and their birthdates printed on the envelope?

No, not necessarily. A violation of HIPAA would be giving out your patient information. You can call the dentist's office and ask them to remove the info from the envelope for future correspondence.

J_9 is correct, not HIPPA. It may be very inappropriate on the dentist's part but not illegal.

Is it a violation of HIPPA or any privacy law when a former dental office sends a letter to our minor children with their account numbers, birthday and our home telephone number printed on an address label? I really don't want the US postal service delivering private mail with our phone # and their birthdates printed on the envelope?
Not HIPPA but I certainly would call and voice my protest - totally inappropriate and possibly dangerous.