Drop your idea of staying on Diani. This must be one of the nicest stretches of beach in the whole area. Ben and family have a little cove which, if you are lucky, you and your group might have all to yourself.
It's a beautiful spot for camping, swimming, eating, drinking and they also have a decent self-catering house that can be booked for large groups or singles or couples.
I stayed for New Years Eve (2012-2013) with a bunch of friends and we had the whole place (and beach) just for us - it was excellent! Ben and family made us really welcome.
It's quiet but it's a short taxi ride to Diani's attractions or you can easily walk along the beach during the daytime to an array of other hotels/restaurants etc.
I would absolutely recommend this place.
If you own or manage Tiwi Sea Breeze Restaurant, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.