Can you explain what you mean re quiet??

Quiet as in people, or quiet as in still water rather than waves??

both debbg

Cala'n Porter, only small our 3y.o. loved it, we got down there every day before 10am and a couple of hours was great, after lunch it tended to get packed.

We really like Arenal d'en Castell because it is quiet, both ways. However, we have been there May/June(half term) so I cannot say what it is like in August. First time we went we had a 1 1/2 year old. We could sit just two meters from the water and watch him play if we wanted. It was not a problem if he wandred of a bit as we could still see him as it was not that many people. The beach is horseshoe shaped bay which makes it quiet.

Arenal beach is beautiful, but most of the hotels are seriously "uphill" from it......you may prefer a flatter resort?

Although the Aguamarina fronts onto the beach......

I found Cala Galdana was just right for my 4 year old, not a quiet beach but plenty of room.

Nice shallow water and lovely soft sand set in a beautiful horseshoe bay.

my favorite beach on the island is cala antandria beach it is a small cove and it is incredibly quiet and there is also sa caleta beach which is even quieter and about 5 mins drive from cala santandria.
it is on the north west coast of the island