Tilly's parents could have responded to her behavior in two ways.:
1) The parents of Tilly didn't take her seriously. They attempted to calm her down thinking she was scared of something. As a consequence, the tsunami waves might have washed them away.
2) They were careful parents, listening to their kid, and taking seriously her warning.
If I had been in their location, I would have listened to her too and realized that a risk was approaching. I would therefore have hurried away from the beach, too.