Car seats next to each other in a 2007 Kia Carnival or 8 seater Van?

Hi I'm deciding where to put capsule and car seat in van, I have a 8 seater carnival and at present my son who is in a Safe-n-Sound Maxi Rider AHR Convertible Booster Seat behind passenger and when our daughter is here in June she be in a strider dlx capsule till she 9kg then we want to purchase aSafe-n-sound Tilt and adjust seat and can be rear facing after she outgrows capsule we want her in the middle seat, now would our son be better behind driver so other people can access back seat from passenger side as its safer. Im confused I need some info please