There are plenty of processes in Newtonian mechanics that amplify small differences. (E.g., in a game of snooker or billiards, it doesn't take many bounces before quantum fluctuations have a substantial effect on where the ball goes.)
Coin flips in particular aren't great, though. A skilled coin-flipper can, I think, get any result they want with close to 100% reliability, which suggests that at least some superficially-plausible-looking kinds of coin flip don't amplify small differences enough.
E.g., in a game of snooker or billiards, it doesn't take many bounces before quantum fluctuations have a substantial effect on where the ball goes.
That's not obvious to me. Evidence..?
E.g. the three-body problem is unsolvable in classical mechanics, without the need to postulate quantum anything.