As far as viewing, especially from a kid's standpoint, a seatback mounted screen would be best, as they would be in line with a kid's line of sight. However, putting a screen(s) in the headliner is likely an easier installation.
I chose the ceiling mount for a couple reasons.
1.Im a lowrider and my seats are bottomed out and tilted back so I can cruise slow and low and the monitors would be in reach of my kids hands and feet plus they are small.
2.The ceiling monitors are bigger and rotate back and forth to adjust viewing angles.
I have a 8.5 in. ceiling monitor and with my kids on both sides of the back seat they have no problem watching the screen.So if you are taking votes mine is for the ceiling.Good luck and keep us posted on what you decide....Nor-Cal