Risky. Invites theft. Ive seen many cases.
And you hv to replace the smashed glass as well.
Ceiling mounted unit:
A more secure choice.
Both above options are not portable.
If your kids cannot be controlled within the vehicle without gimmicks, its forseeable they're also difficult to keep quiet once you reach your destination.
In modern times, the most effective parent is the Ipad.
Perhaps you can buy each of them an Ipad. Portable, and every family member can use it.
Oh 1 more thing...get a pair of decent earphone or headphone.
Once dear junior is enveloped with nice sounds from the Ipad games....it'll be blissfull for you.