Tsaunamis are rare events (thank goodness)...
Not everyone knows that a last minute warning sign is when the water suddenly recedes. If the adults had never seen a tsunami and had not learned about their characteristics, then they wouldn't know.
Several years ago, I heard a comedian talk about how a tsunami warning attracted a bunch of people to the beach to "check out the tsunami, dude." Fortunately, nothing happened (if the story was even true) or we would've had a mass Darwin Award for the year.