There is no single answer: it depends on the day’s sea more than on the beach. But with children, three things decide: sand or pebble, services nearby and shelter from the swell.
Playa de la Arena
Black sand, a promenade and services within reach. It is the most comfortable for a day with children in the municipality: parking close by, places to eat and easy access. As on all this coast, check the flag and the sea state before swimming.
Playa de Los Guíos
Spectacular, at the foot of the cliffs, but pebble and deeper: it is a launch point for kayak and snorkel more than an easy beach for toddlers. Fine for children who already swim on a calm day; for the youngest, the Arena is easier.
The rule that applies to all
Check the sea before you set off
A day of heavy swell changes the plan on any beach. Check it first.
The flag rules
The beach flag overrides any forecast: it is set by whoever is there watching the water.
Sand for the youngest
With babies and first swims, the sand of Playa de la Arena is easier than the pebble of Los Guíos.