For anyone wondering, assuming linear acceleration that's roughly 1 g. In a rear-facing seat, that's 1 g eyeballs-out, which should still be perfectly safe for the child, but possibly not for the cleanliness of the seat.
...and if you're wondering what 1g eyeballs out feels like, you can always try looking at your feet.
Shouldn't it be actually ~1.4142g at a 45 degree angle?
You're proposing that gravity is stronger in a direction other than the one straight down to the center?
I think they’re referring to the fact that in the Ferrari, you’d experience both 1g eyeballs out plus the 1g of normal gravity, for a total of sqrt(2) at 45 degrees, when compared with looking at your feet producing 1g down.
Also, basically any boring passenger car can brake at or slightly above 1 g.
Maybe if the acceleration is smooth enough, but the sudden appearance of a new g yanking your head to your chest might be dangerous for a baby.