What are these carseat for ? WHY do thay have that thing ?

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They are absolutely not seen as safe!! Your kid can slide over or under the pillow and it can make internal damage as it presses on the soft part of the belly. In Norway and Sweden they are "red listed" and not adviced to be used at any age! 
Also at 3 years old your kid should rear face at least a year or three more. As there's no way a three year old is big enough for a three point harness- Look at Britax Max way plus, klippan opti 129, axkid minikid.

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PLEASE do not use these for infants. Infante really need a five point harness. (You can pretty much install any new car seat with the latch bars in most cars. That’s not what this is doing. It’s replacing the five point harness (the one that typically goes over both shoulders to a chest clip which then attached to a to another clip that is around both hips). 5 point harnesses are much much safer for infants.

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That is instead of seatbelts. Studies show, that in a crasch, it is as good as a seatbelt and it prevents the injuries that seatbelt makes in a crash. Therefore in some cases it is even more safe option, but can't remember exactly, maybe someone else can tell you exactly.

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Hey, I used to have a car seat like that when I was about two, after outgrowing the infant seat. Then I went straight to sitting in the bench seat and wearing a seatbelt when I was about 4 years old. 
... Please don't get this kind of car seat.

What are these carseat for ? WHY do thay have that thing ?

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