Car seat advice

[snufflepuff]

What happens when a child's head starts to stick out over the top of their car seat? I'm talking about forward facing seats with a 5 point harness.
One of my parents has been talking about buying a high backed booster without a 5 point harness for her LO (29 months) because her head sticks out over the top of her car seat.
I have 2 car seats that are suitable from 9 months to 12 years, and one that's from birth to 4 years. The LO in question usually uses the birth to 4 years car seat and her head is very nearly at the top. She's not particularly large for her age at all, so i'm unsure as to why she has almost outgrown a seat that should be okay until she is 4! The situation is the same for my own son- he's the same age, very slightly taller, and his head will soon be over the top of the car seat. I've seen lots of children in car seats with their heads sticking out over the top, so i'm not sure if maybe that 'rule' isn't very well known.
Do I now need to go and buy another 9months to 12 years car seat? These have adjustable side protection so I can move it up as the child grows.

[karen48]

My husband has been on a training course to fit britax and graco carseat and the rule is that the child is ok in the seat until there eyes are level with the top of the carseat. Britax do a seat that is used on a full harness and then as the child grows you can then tuck the harness away and use the seat with a seat belt and it has a adjustable headrest. This is one of the best seats i have brought and over time when my other seats need changing i would replace them with this one .karen:)

[snufflepuff]

Ah, so eye level, that makes more sense.
Is it the britax evolva 123? We have one of those and I agree, it is fab. My son certainly prefers it!