Virtually all the easily-accessible beaches are likely to be quite busy during good weather in holiday time.
Porthchapel beach, below St Levan's church, at the end of a cul-de-sac from Porthcurno, is going to be quieter than most, because you cannot park near it, so walking is the only way to get there. There are NO FACILITIES there, just a path and the beach, and the cliff path going west to Porthgwarra and Land's End, and east to Porthcurno, Treen and the Logan Rock, and Penberth Cove.
Porttcurno is much larger, has car park (probably paying), refreshments, toilets etc. And a village shop I think, the Minack Theatre, and nearby small hotels. 'First Bus' has a regular service from Penzance to Porthcurno and Land's End; they use open top buses in good weather.Portcurno was the site of the first trans-Atlantic cable (in 1870), so it was in at the birth of modern telecommunications, and there is a Telegraph museum:
telegraphmuseum.org/
It is £22 for two adults and three children I think.
[Very annoyingly, I haven't yet managed to find the parking charges for Land's End.]
Where are you staying, as that could influence where your nearest quiet beach is?
Treen and Logan Rock have rocky beaches, and nice walks. Parking, pub, and possibly a shop at Treen.
BEACHES:
www.cornwall-beaches.co.uk/west-cornwall/porthchapel.htm
Pedn-Vounder may be difficult to climb down to, but worth it if you can manage it!
www.cornwall-beaches.co.uk/west-cornwall/pedn-vounder.htm
www.cornwall-beaches.co.uk/west-cornwall/porthgwarra.htm
www.cornwall-beaches.co.uk/west-cornwall/penberth-cove.htm
[Fifty years ago we used to know every inch of these beaches, staying four or five weeks in nearby cottages! The memories these sites bring back!]