Thanks for the thoughts everyone. I did check Pennsylvania's traffic code as RustyRatRod suggested, it seems no matter what the seat belt situation is in the car, a booster seat is required for someone her size.
Is she small enough to ride is a full seat that has its own harnesses? That would be ideal. My boys, 8 and 11 are gigantic compared to most kids their age, so no seats for them. We buckle in and go for a ride, but most around the neighbor hood.
She has several boosters, most just the bottom piece, but she has one that's high-backed that I switch between the Jeep and the Caprice. I'll probably use that in the center rear seat position, but no matter what booster we use, it's still just a lap belt holding her in.
It's gonna be a looooong time before my 1-year old son gets a ride in Daddy's new project :lol: