Which is the best positioning?

[Unregistered]

i have a 3.5 yo in a ff conv. seat and a newborn in a rf infant carseat. i have three seats in the second row. baby or toddler in middle? left or right side for other child? thanks!!!

[Splash]

The best positioning is where you can get the best install for both seats independent of each other. If the installation of each one is comparable, then the older child should go in the middle since he is FF and therefore in the least protected restraint.
As for which side the other one goes on... again, best fit. If it's all the same, whichever one you prefer.