In reply to notaclue:
My first bit of advice for camping in Cornwall is to find a cheap cottage or flat........
The problem is that there aren't all that many campsites right next to - and on the same level as - the sea. I can only really comment on the north coast and then Penwith. Hopefully, somebody who knows more about the St Austell-Looe area and the Lizard will be along with better suggestions.
Avoid Newquay. At all costs - what an armpit.
West of Newquay, there's a Haven park at Perranporth. Plenty to do, but it's about 250 feet above the sea (PITA with beach gear) and most of the vans and the camping area are set back quite a way. It's extremely chavvy, which the kids will love.
There are a couple of small campsites around St Agnes, but they're set back quite a bit - too far for little legs.
Hayle has sites, including statics and camping - look around the Godrevy area - and whilst they might be set back a little, it's largely level ground so an easy walk. They tend to be big outfits, and the independents are sometimes hit and miss as to whether they open. It's really not a bad option, though - close to A30 and well-placed for Penwith, St Ives, the south coast and supermarkets.
St Ives is going to be rammed. Best to stay elsewhere and travel in on the train from St Erth.
North coast there are some tiny independents and I'd do a UKC search, as everybody has their favourite spot. Botallack is probably the most frequented climbers' site, just outside St Just.
+1 for Trevedra, although it's another one with a steep path down to the sea. Beautiful spot, but very open if the weather's unkind.
Treen has a popular, basic site which is near Porthcurno, reasonable flat walk to the edge but then quite a hike up and down.
Treverven is ace for St Loy, but not for the beaches.
That's about where I run out of steam, so hopefully a Lizardite will be along soon. They have electricity now, so one of the villages might have discovered the internet
It depends on whether you're looking for child-friendly (clubs, play park, pool, amusements, ice cream - ie what kids want) or child-friendly (camp in a field, no electricity for Nintendos, cold showers - ie what they're bl**dy well going to get).
Hayle's not such a bad option, particularly if you head for Godrevy. The beach is good and transport-wise it's well-connected, but it's perhaps a tad scruffy inland. Trevedra is beautiful, but isolated and a bit open.
HTH
Martin