There should be an exponentially increasing delay for such a system, so that the phone would make you wait hours (or days) before letting you make your 10th gue

There should be an exponentially increasing delay for such a system, so that the phone would make you wait hours (or days) before letting you make your 10th guess. That would require the 5 year old to not get bored of the useless phone, and the owner to not find the phone (and enter the correct code) for those days too. Also, it would make sense to include a simple proof-of-intentionality system, like the old Nokia keypad unlock feature to prevent pocket dials. The phone could prompt you to type a displayed 4 digit code before typing your actual PIN attempt, for example.