I’ve always looked at the coastal path North of Hayle as a fun looking proper ride. From Godrevy round to Hell’s Mouth and onwards there are some nice sections that would be easy for a family ride but it’s interspersed with narrow rocky sections which run close to the cliff edges.
For that reason I wouldn’t take my young family up there on bikes.
Go and have a look though, walk a part of it and see if it shapes up to your expectations. You’ll be able to park for free if you drive up past Godrevy and pull in at the little Hell’s Mouth cafe thing.
Then there’s always the coast path with out of St Ives but you’d be better off leaving that to Chris Akrigg to ride ;o)