Shortly before the baby arrived I put my oldest (just shy of six) in the booster variation of her Frontier. I thought it would make my life easier to have her in the "way back" of the minivan and figured she could handle the responsibility. She was in this seat for right around five months now...and it felt like every single time I caught her glimpse in the rear view mirror she was slouched down, leaning over to talk to the baby, putting her feet up on the shoulder of the seat in front of her, etc. After a few million reminders to sit up and WHY she needed to sit safely and a few sharp pulls off the road (thinking maybe stopping suddenly to remind her that the car can't move when the kids aren't safe) I finally made the executive decision to re-harness the seat. Since she's now six it's not an issue for her to fully harness herself and pull it tightly.
Clearly she's not ready for long-term boostering...
Which got me to wondering when most kids are truly ready. Now, my daughter is a mover and a shaker. She can sit still for just a few minutes before she's wiggling and moving. She's the kid that does cartwheels down the street instead of just walking to the bus stop!
At what age were your kids able to sit safely in the booster? Did you give them options to be boostered and then opted they weren't ready yet?
Luckily my kids know that I'm nuts and don't see it as unusual to still be in car seats RFing until eternity. I'm still amazed at how many of my oldests' friends (mostly old 5s, young 6s) are in nothing more than belt-positioning boosters.
I've been wondering about this and am eager to see people's responses. We let DD1 use a booster during a vacation last month. She just barely fit the minimums listed by the manufacturer (4 years old, 40 lbs, 40 inches) and a booster is what was available. It was clear though that we need to get a harness booster when she outgrows her current car seat (which I think will happen when she has one more growth spurt; she's very tall). She couldn't stay still or stop messing with the belt.
I've been wondering about this, too. DD is 5.5 and we recently had her use a high back booster on vacation. She did surprisingly well in it, but she's not really a squirmy kid anyway. But most of our drives were pretty short so I don't know that she got a real test. Ever since we returned she keeps asking me to convert her seat to booster mode. I am very tempted because in some ways it does seem easier, and we occasionally ride with other people. (She uses a Ridesafer travel vest on those occasions, but I really don't love using that.) But she is not even 42 pounds yet, and I don't know...the five point harness just seems safer.
So I realize I'm totally not answering your question, but hopefully someone will chime in with more definitive signs of readiness.
My son is 5.1 and wiggle worm. He's big though at 46" and 44lb. I decided to start booster training because I wanted him to be able to go in friends' cars after school and I didn't trust anyone else to install his nautilus (plus its heavy!). I got him a turbobooster and explained that we were going to practice using it. He's so excited about his new seat and takes it very seriously which is a surprise because he normally brushes off safety concerns. I've only taken very short trips though and I've stressed that the nauti is still his main seat. We're taking a trip on Friday when we'll be driving late at night and I will be very happy to have him harnessed for that.
So basically I think that booster readiness is a combination of physical size, emotional maturity and motivation.
DS was definately booster ready *emotionally* before 3. I am not sure about DD because we didn't have boosters when she was that young (we had them when DS was that young because we had them for DD). That said, both of my kids have really internalized safety rules (they both get upset on things like fieldtrips if kids aren't sitting properly in cars or out playing if helmets are on wrong). By that time he was long past day time naps and would sit correctly. DD was boostered before 5 with no problems (of course, that was with the Britax booster that had belt lock offs which removed a lot of squirmy issues).
With my experience with other kids, though, I would say this was VERY abnormal. On recent field trips (for 8-10 year olds) I have had at least one kid in the car who did not sit, IMO, safely without a booster (this was not a size issue, but simple maturity/lacking of rules)--- leaning way forward, trying to turn sideways, etc... I think most kids can be booster ready by 5 if the parent is willing to make the rules clear and stick with them. I often ride with a friend who's six year old should still be harnessed, IMO, simply because she never says anything to him about slouching down and putting his feet on the seat/ leaning out of the seat/etc...