Well, my "familiarity" with the place is about 25 years out of date, so I doubt if any recommendations I may make would be much use to you
When I was last there (05) we stayed at Alexander Bay.. the shire (I think) had set up a new campsite there complete w/- composting dunny, & we had the place to ourselves, and thought that it was quite OK
Thomas River has completely changed, w/- gravel road, & "organised" (pay) campsites, which looked OK too, though we didn't stay there...
When I was living in the area, we used to just drive down to the beach (sandy track), pick any spot that suited us, and that was that...
Hoping to get back over there late next year (No 1 son lives in Perth), and we'll more than likely make a stop-over at Seal Ck or Poison Ck, then have a look to see if we can get to Israelite Bay (we'll be towing)...
Without any up-to-date knowledge of the area, I'd say either Seal Creek or Poison Ck would be the go for what you're planning..
I will make one suggestion though...
Go for a drive down to Kennedy Beach (track is off Merivale Rd, directly opposite Exchange Rd)... To the east of the track, just behind the cleared farmland (my old workplace

), there is a freshwater lake... You can zoom in on it on Google Earth).. the track(s) into the lake itself look to be a bit overgrown, so may take a bit of finding, and you may get a "massage", but if you can find the track into the southern end of the lake, you'll find a nice white sandy "beach" where any kids will enjoy a paddle... this will depend how full the lake is of course, but should be OK this time of year..
Don't tell anyone I told ya, but there used to be critters in that lake.... 6 letters, starting with a "m" & ending with a "n"..............
I see no reason why they shouldn't still be there
Have fun....
:cheers: