When did you move your LO to one of these? Can you recommend yours? TIA!

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Re: Booster seat with back
I'll move him in middle school.
(see post above).
Seriously - I'm in no rush. I'll change out his seat when he out grows the 5 point limit and not until.
We've got the Graco Nautilus which converts from 5 point to high back booster to bottom only booster.
Right now they're both in Britax's and we use the Nautilus as a 2nd seat in Daddy's car which he rarely rides in but that's the high back booster we'll be using when the time comes.
Total score: 6 pregnancies, 5 losses, 2 amazing blessings that I'm thankful for every single day.
We started transitioning my daughter a few weeks ago. She's tall and all torso, so she's about an inch of torso growth (6mo-year) from outgrowing her radian so it's time. So far she has been perfect in it, has never wiggled out of place or played with or moved the seatbelt.
The minimum recommended stats for using a booster are 4 years and 40 lbs, but the more important factor is that the child can and will sit correctly 100% of the time they're in the seat. The booster puts the responsibility of keeping the kid safe on the kid instead of on their parent, so the child has to be ready for that. I decided to test the waters and my daughter has done perfectly so I let her ride in it. My younger daughter is far less compliant with rules so I have a feeling it will be a long time before I consider putting her in a booster.
We have an evenflo big kid, which doesn't have a great reputation for fitting kids well, but positions the seatbelt beautifully on her. With a booster you have to find the one that fits your kid and your car well.
When DD was 4 and 40 lbs we began to allow her to use a high back booster in rare circumstances. So far she has used it once in her dad's car to go to "bring your kid to work" day and another time on a weekend trip to visit family. She is still in a 5 pt harness in our everyday car and will remain so until further notice. We don't have a plan of when we will transition her, as for now she still fits in the Britax Frontier with a 5 pt harness.
ETA: For our high back booster we have the "Graco Turbobooster." I like it but honestly the 5 pt harness just feels more secure.
All of this. Being booster ready has less to do with age and weight and more to do with maturity and the ability to stay properly positioned within the seat at all times. Many children do not reach that point until five or even six years of age. For us, I will keep my children harnessed until they start kindergarten at age six, provided that they are ready to begin booster training. Note I said booster training. There's more involved in putting a child in a belt-positioning booster seat that sitting him or her down in it and pulling the belt across. It's a learning process that should be taken seriously by child and parent.