We have five carseats in ours. We have 3 across the middle and 2 in back, side by side. We keep them rear-facing to age 4. I always like to use the middle seat since it's safest. I also like the middle row best since it's the actual middle of the van, hence away from point of impact in front or back.
You can do a booster for a 5 year-old but that's quite young. Is she 40 lbs? Mature enough to sit properly in one?
I also like having infant seats outboard since they are easy to click into the base that way. Right now our 3.5-year-old is in the back rear-facing, and we switch spots as needed based on our needs. Soon he'll go forward-facing and the current toddler will get his seat.
Good luck with your setup!! You just have to find what works for you. :)