We recently spent a week at the Oakdown Holiday Park in Sidmouth, South Devon. They advertise it as "The perfect place for caravan, touring, camping and glamping holidays..." and we have to agree. It is very family and dog friendly and beautifully maintained. As if that was not enough it is just a 20 minute walk down a lovely path to the Sidmouth Donkey Sanctuary. This was the highlight of our week there.Robrace have you had a look to see if there are any THS in dorset?
I like oakdown near sidmouth but it can be a bit pricey but its nice
Good luck finding somewhere i love dorset
You can see a a video we made of the site here: