I suspect these don’t count for legal purposes, though.
At least MA requires that kids under 8yo: “be fastened and secured by a child passenger restraint”. And defines that as “a specifically designed seating system which meets the United States Department of Transportation Federal Motor Vehicle Safety Standards, as established in 49 C.F.R. 571.213, which is either permanently affixed to a motor vehicle or is affixed to such vehicle by a safety belt or a universal attachment system.”
The positioner you linked doesn’t say it meets federal standards, so it probably doesn’t? Compare to, say this inflatable booster that says it “exceeds all US federal car safety standards & European regulations (FMVSS213 & R44/04)” or this belt positioning folding flat seat that says “Meets or exceeds NHTSA standard FMVSS 213 in the USA and is regulated for use in every state.”.
At least MA requires that kids under 8yo: “be fastened and secured by a child passenger restraint”. And defines that as “a specifically designed seating system which meets the United States Department of Transportation Federal Motor Vehicle Safety Standards, as established in 49 C.F.R. 571.213, which is either permanently affixed to a motor vehicle or is affixed to such vehicle by a safety belt or a universal attachment system.”
The positioner you linked doesn’t say it meets federal standards, so it probably doesn’t? Compare to, say this inflatable booster that says it “exceeds all US federal car safety standards & European regulations (FMVSS213 & R44/04)” or this belt positioning folding flat seat that says “Meets or exceeds NHTSA standard FMVSS 213 in the USA and is regulated for use in every state.”.
Julia linked me to http://ridesafer.net/ridesafer/ which does say that it meets federal requirements, though is much more expensive
For what it’s worth, I own one of these and I like it.