My only real concern is that with the the Navi CPU located in the rear passenger side AND the Sub-Woofer located in the rear driver side, in a rear collision scenario the rear crumple zone may propel these units towards the kids. I've attached a shot of my kids in the rear seats for reference.
BTW, I would definitely NOT put and toddler younger than 4 years old in the rear, at least not until they know how to obey instructions and can sit still in the car properly! Having them jumping about in the rear whilst driving is one experience I never want to go through again!!!