The center of the rear seat is usually safest since it is farthest from a possible side impact, but only if your carseat fits well in that position. Any position in the rear seat is acceptable unless prohibited by the vehicle or child seat owner’s manual. The seat behind the passenger may be slightly safer than the seat behind the driver, since it allows you to unload the child on the curb side, allows you to see your child more easily from the driver’s seat and is very slightly less likely to be hit on that side in a side impact. There is also a small risk that a front seat-back could collapse in a severe rear-end crash, and the passenger seat is less likely to be occupied by a heavy adult who could crush a carseat in this situation. The front seat is generally not advised for children 12 and under, especially if an active airbag is present.

If you have more than one child, the safest arrangement may not be obvious. Usually, it is preferable to put the child with the most protection in the outboard location; rear-facing seats offer the most protection from side impacts. These types of questions are very appropriate for our forums.”

So the safest place is where it fits the best – with less than 1 inch of movement. If it fits equally well in all positions then the safest place is in the middle.

http://children.webmd.com/news/20080507/safest-spot-for-babys-car-seat?src=RSS_PUBLIC