We live nearby too. The rock centre at Chudleigh has a tree walk but so does River Dart - like a Go Ape for 8yos up. my dds did it last year there. I like River Dart, more worth it than some of the other overpriced "visitor attraction" rip-offs in the area.
Blackpool Sands, I'd say sniffily, is for people who don't really like the beach. It's pretty, pebble beach, with neat sandpits for the children. and a good organic cafe. Not good for watersports or small children to paddle in as it's quite steeply shelving, as pebble beaches tend to be.
Anywhere around Salcombe/Hope/Thurlestone/Bigbury is much better for proper beach.
Dartmoor has lots of very nice places. If you want to entice the dc, can you combine a walk with one of the places for swimming/paddling in the river. Spitchwick, near River Dart country park, is very popular, you can take rubber rings and bounce down the rapids. Or you can go to Fingle Bridge, which has a good pub and picnic area where children can paddle. and from tehre you can walk to Castle Drogo.
Or you can go up Hay Tor which is a very short walk but feels like a proper hill. Near Bovey Tracey.
Stover and Decoy country parks are both nice, I thikn, for gentle picnics/cycle rides/child-sized walks. Stover has a poetry trail and tree level walk, Decoy has fountains for playing in in the playground. All of those things are free and my children enjoy them.