Hi Tigermoth. I live down that way.
The Hallsands hotel isn't open, it's all boarded up and looks very spooky standing abandoned on the edge of the cliff. Went in there when it was still open about 2 years ago and it was a bit like The Bates Motel
I'm pretty sure that the lighthouse at Start Point is open during children's holidays so you can go up into it. My ds got a badge for making it to the top quite recently.
From Start Point you can see North Hallsands and the edge of the lost village that was swept away by the sea in something like 1907. The Trout Appartments are there: they were built, by hand, by the 3 Trout sisters after the village and their hotel was swept away by the sea. 2 of them died and the third kept the hotel ready for guests with all the tables laid for dinner until she died (in the 60s I think). Another spooky tale! You can drive there in 10 minutes from the lighthouse, although there's not too much to see. It intrigued my ds and his friend though.
I don't know any play areas in Totnes I'm afraid, but Slapton Sands, just past Dartmouth, is a lovely long stretch of beach and the Start Point Inn at Torcross, at the end of the beach has a family room and welcomes kids. If you go to the car park opposite (by the tank, on the corner on the bend) there are loads of ducks and swans to feed.
There are 3 play parks in Dartmouth; one on the front, in Coronation Park, one by the main car park and one in Victoria Rd on the hill. Coronation Park is the best one and has a kiosk selling drinks and stuff as well as a football net and swings etc.
The steam train at Kingswear (ferry from Dartmouth) takes you to Paignton, but you might want to come straight back again! Not a nice place, although the train itself is fun and you get a pretty ferry crossing.
HTH and you have a great time.