It makes perfect sense to me that RH was taken into custody at the scene. Look at the situation dispassionately for a moment.
There is a small child victim in full rigor mortis, with obvious skin discoloration, open eyes with protruding tongue, visible injuries to the child's face, and a strong scent of decomposition, and the fact that RH had been DRIVING the car moments before with such a gruesome victim situation inside. To any LEO, they would immediately know that the time of death was anywhere from a few hours ago, up to 24 hours-- definitely not immediately recent (as in the last few minutes or hours).
Even before they knew about RH's "double life", bystanders and LEOs report how very strange RH's behavior was at the scene. The officers definitely could have been thinking that the child was dead from abuse or foul play, or even staged post mortem into the too-small carseat to make it look like a heat death. (And yes, I think the LEOs at the scene probably immediately recognized that the bucket style carseat was for an infant, and the victim was clearly a toddler.) RH being on his phone while others tended to his child, as well as him cursing at the LEO when she asked him to get off the phone only adds to the big picture. I can definitely see why they took him into custody at the scene. Just nothing about that scenario looks like an accident, IMO. I'm actually surprised it took LE until 10 pm or so to officially arrest RH and read him his rights.