They're just guidlines, I think. I don't believe a cop would pull you over and measure your kid and give you a ticket because he was three inches too short.
It's really just there because kids aren't quite the right size for seat belts to fit them properly until they're bigger. And this thing doesn't really look like a car seat. It looks like someone yanked the front seat of my car out and put it in the back seat. It's almost big enough for me to sit in, and I'm 6' 2" and 260 lbs.
I also don't think it'll be too embarassing for him. The other day I had to drop my friend's 6 1/2 year old at school and I asked him to sit in it just to see how he fit. He still had some room to grow, plus he was pissed off that he couldn't stay in it.
We got these because 5 point harnesses are safer than just using a booster seat and the car belt.
And what problem did people have with you playing by yourself? I'd give my right arm if my kid would play by himself. Amusing a 2 1/2 year old for hours on end is fucking tiring.
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100 pounds of shit in a 25 pound sack.