Evening, sorry just seen your post. I dont know of a campsite in lulworth cove but i know of one round the corner, just outside of swange.
Its a excellent quaint site, out of the way but still within walking distance of good pubs and coast path.
Its Toms field Campsite, (www.tomsfieldcampsite.co.uk
) they dont take booking for one nights, you have to turn and see. But wortha look.
Hope this helps,
P.s Are you insane lending it to the inlaws, your braver then me..........