Cornwall in May for 4 nts - cheap and toddler friendly

[Belgrano]

Hi
Can anyone help with recommendations for a place to stay in Cornwall in May. We don't mind self catering or a hotel. Sands and Bedruthan resorts are v expensive and only let you go for a week which we def can't afford.
Lots of the cottages like the StMoritz complex are already booked up (we have to go the 2nd bank holiday week which starts Sat 23rd but can go on any day for 4 nts).
Ideally we wanted to spend under £500.
Any ideas? Thanks!