I'm in the exact same situation as you, except I'm due sooner
We drive an Odyssey and, like you, will have 2 in boosters, 2 in 5 pts, and the newborn. We're going to put the two boosters beside each other in the back row with the Radian on the outside (so I can easily reach for buckling). Then the new baby and other 5 pt seat will be in the second row.
I went to a car seat clinic and fortunately they had a Radian I could test for fit. The back row did work with my turbo boosters, but I found an even narrower seat (the Harmony booster) that has a low profile and will be very easy for the kids to buckle themselves in.
Sorry I have no advice to give! I drive a Mazda CX-9, but I only have one baby seat and one booster for DD. I love all the new stuff I read on here though! Good luck... and BTW you might want to stick with your Sienna as SUVs are CRAZY on gas milage! I had an Escalade EXT (back when I only had two kids lol) and I think I got about 13 MPG with that thing!
Krystal & Donovan - 12/2/06
Reagan - 10/2/02
Maximus - 3/10/05
Liberty - 12/11/08
My angel in Heaven 1/7/13
I will be checking out this harmony booster as well. We have two captains seats in the middle row, so that is where one of the 5pt. seats will go and also the pumpkin seat for the baby. I agree with pp that gas mileage would be terrible in an suv. I am thinking if we did end up with a 6th we would trade our current sienna for another sienna with the extra seat. Figuring all of this out does make me feel at ease. As much fun as it would be to drive a big SUV, with gast at 4.00/gallon, I think I would be stuck at home! Thanks for all of the input!
I don't know that we are even going to have a 5th as of right now, but we may need a new vehicle at that point. We have a very old Voyager, I think. lol. It just has a lap belt for the middle seat in the back. My daughter is almost 10, but on the small side and prefers her booster, so she is still in it. My son is 7 and Utah law says booster until they are 8, so he is in one as well. And then the twins are in the convertible seat. We have one baby in the back row and one in the middle row, because I was tired of the older two fighting. lol. It is such a pain to get back baby in and buckled and such. I should move them back. Our van does have the built in car seats as well.. so I like having a booster over one of those in case I need to transport an extra kiddo, which sometimes happens.
Rachel, momma to 4
dd 9, ds 7, twin boys Dec 09
I nursed my twins for 2years and 2 weeks! A little sad to be all done now.