There are narrower booster seats to be purchased. Even fractions of an inch may make a difference.
The main reason for the boosters is to prevent the 3 point seatbelts at the neck.
If the child is in the middle, where there is only a lapbelt, they may not need to be in a booster at all. Definitely check local laws for that.
In our state, it's either 8 years old or 80 lbs (your choice, go figure) of when they don't need to be in boosters.

Let's see...growing up, my parents used to let me ride behind the back seat of a Beetle. It was nice and warm just above the engine.

When they bought a Corvette in '76, we were put in the back with our faces inches away facing the rear window and our feet straight back over the spare tire/fuel tank area.

My brother has that car now. Very cool. Different times.
Different and fewer drivers, too. No cell phones/gps/videos and people generally aiming there car in a direction and then correcting at the last moment while they fiddle , which I see a lot of.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."