South Devon stream railway at Totnes, also Rare breed centre ( next to train station) and at other end of the line Otter and butterfly centre at Buckfastleigh. All good visitor attractions that are Ok in the rain.
Pottery painting in various locations including Totnes. maybe just a hand print from youngest but 4 yr old should like it.
Pennywell has plenty of indoor and outdoor stuff to do. Farm theme fun park.
Soft play at Torquay and Newton Abbott and Teignmouth (IIRC) Swimming pools in various town, leisure pools at Torquay and Plymouth Pavillions. . Pier for old fashioned seaside slot machines at Teignmouth, also beach and FAB playground next to beach.
Try Pennywell Farm - great fun or an hour away the Miniature Pony Centre which my girls (3 & 5 yrs) love. Both have indoor areas so you can get out of the rain - and if you can brave the westcountry mizzle then there is loads to do!