We love our Diono Radian. Can be a little hard to install but it's built like a tank and is narrow so fits well in the back seats.
Car seat or booster? For car seat, Diono Radian RXT. For booster, this $22 Cosco seat at Amazon works great: https://www.amazon.com/Cosco-Topside-Booster-Car-Seat/dp/B006ZISVSO
recommend a high back booster for a 4 year old and switch to a seat only booster as child gets older. https://www.amazon.com/Graco-Affix-Booster-System-Atomic/dp/B00AHVR7ZI/ref=sr_1_3_a_it?ie=UTF8&qid=1522779456&sr=8-3&keywords=high%2Bback%2Bbooster%2Bwith%2Blatch&th=1 also you are in california so look at this. Child Safety Seats
I have the Recaro Performance Sport seats which are a great combination seat but I wish I had a narrower seat like the Diono.
I've never used with a seatbelt extender, but might be helpful if you're trying to put 3 side-by-side.
Does the radian sit flush against the back and bottom of the seat? Our Diono r100 does not so I am looking for something else. Our 6 yo still falls asleep in the car and since it's not safe for her to slump over, I really want something with a back but haven't been able to find anything.
Note that no car seat or booster has been crash tested with seat belt extender and it's just another point of failure, so I believe most technicians (and the Tesla S manual) will tell you not to use them.
If your 4 year old didn’t outgrow the forward facing seat yet, it’s better/safer to keep him/her in it as long as they fit it (weight limitation of the seat)
If you’re looking for a narrow backless booster that fits the Model S (and does not cover the buckle receptacle), I would recommend the Cosco Rise booster. It is a new booster and it is only $10 at Walmart. My kids didn’t like the bubble bum because of the additional steps involved in securing that seat, and this Cosco is a lot easier. I also tried the Harmony youth booster and that too was better than my former standby - the Graco turbo booster - but not as narrow as the Cosco. And for me, a seatbelt extender seemed too risky given the lack of testing and the fact that most of them are not sold by major companies.
I have both - the Diono for my 5 year old and the Cosco for my 9 year old. Wish I could still convince my 9 year old to ride in a real car seat - as safe as the Tesla is, I still feel better about having him in a fully enclosed seat. If using the Cosco, put your kid on the drivers side as the car gets angry/confused when he would accidentally plug into the center buckle when on the passenger side.
I picked up two of these today for my four year olds. Still a little hard for them to buckle themselves in but hopefully with time they’ll figure it out. I had to return the graco brand booster seats since they have a wide base and cover the seat buckles. I see people in the forum suggest using a seatbelt extender. I for one would never use something like that unless it was approved for car seats, which none of them are. I also see a post with three car seats in the second row. The owners manual says not to do that as well. Chicco® KidFit™ Zip Luxe 2-in-1 Belt Positioning Booster Seat in Dolce Limited Availability Attention California Residents Chicco® KidFit™ Zip Luxe 2-in-1 Belt Positioning Booster Seat in Dolce
Anyone have more current recommendations for the S? We really want one with a back and using LATCH. New car comes Thursday so I'm a bit late thinking about this now!
It’s been a few years now, but we really liked our cybex booster with z-fix. Very nice stuff. It has a metal clamp that held it in place to the latch anchors so it wouldn’t flop around when no one was sitting in it. The kid still used the seatbelt. And, please don’t use seat belt extenders with car seats. I bought one too, as it seems easier, but then I read some horrifying stories of kids getting unclipped and hurt in accidents. Totally not worth it.