Narrow child car seats so an adult can sit in the back?

So, I have a simple question for all the parents with little ones riding in the Prius... which narrow car seat/booster (with back and head support) do you own? I need help with suggestions!

Recently, we gave up our '11 Sienna XLE AWD in exchange for a '14 Prius III so that we could economize on our monthly budget. With car payments AND gasoline, we are saving over $700 a month, which is a big chunk of change for my wife and I especially because we just bought a house and renovated the entire thing... and automated the entire house... and buying furniture and appliances... Lol. You get the picture.

While it's not a big deal right now that we downsized our family car (BIG TIME), it might be a problem later down the road (pun shamelessly intended) when the weather gets warmer and we decide to go on long drives.

In our old minivan, my wife (5-6", petite) would move to the third row to play with our kids once they get bored with their tablets and movies on the headrest monitors. In the Prius, the only way to do this would be to pull over at a rest stop, let her sit between the car seats, play with the kids and stay there until the next rest stop so she can move back to the front and get some rest for herself.

So my question is, which of the narrowest, fully tested, high-rating, awesome protection car seats can I get?

My daughter is 3 yo is about 40-42" tall, but is also petite like my wife, and weighs about 32-35 lbs. My son, 4.5 yo but a little over 4' tall and is a little stocky, and weighs close to 50 lbs.

The current car seats they have are one of those rear-facing ones that converts into front-facing car seats that have LARGE bases, fully padded, and seats very high (my son's head goes past the back rest by quite a bit... a lot). I've been wanting to replace these to the narrower ones that convert into booster seats for a while and now would be the perfect time to do so.

Thanks in advance!

 

Thanks for the response. Looking at the specs, I don't think my son can fit in this car seat. The seat is almost as high as the one on their car seats right now. Pretty soon, his head will touch the roof of the car. I need a lower booster with enough back support for when he falls asleep.

And to answer to your post, if I look at the rear seats right now, the space between the two car seats is about the width of the arm rest. If I can just take off maybe 5-6" from either sides, my wife will fit.

UPDATE: I'm seeing some on the Babies 'R Us website that there are 15-16.5" wide seats. Are these really the narrowest? I mean I don't want my kids to be uncomfortable, but I just want to find the right ones so I don't have to spend money again if I find out there are better ones for my kids while in the Prius.

The Britax Parkway SG Booster Car Seat looks good.