A day with kids at the shore goes sideways fast when nobody has agreed the rules. Pick your group and activities — get a plan you can print and hand to every grown-up.
Before the first bucket of sand: pick a single, obvious meeting point (the flagged lifeguard tower, a distinctive beach entrance) and teach every child to go there if anyone gets separated. Put a paper card in a toddler's pocket with your phone number, and give older kids a cheap whistle. A child who can't see their parents should stop and ask someone in uniform, not wander the beach looking.
Toddlers need an adult within arm's reach at all times, especially in the water. School-age kids stay in pairs and within sight. Teens get more freedom but still check in at agreed times. The ocean does not care how independent a 13-year-old feels.
Show the children the wet, darker sand line and explain it moves. A simple rule: when the wet line reaches your towel, it is time to pack up. This turns "the sea is coming" into something they can watch for themselves.