Try Tredethick farm. http://www.tredethick.co.uk/ We stayed here twice and trekked over from Ireland for it when our kids were younger. But it was worth it. Felt like a real holiday for us and there was so much to do onsite that the kids loved(and for us too). It is expensive in peak season though, we stayed in shoulder season as the kids weren't in school. Accomodation is lovely too. They do feeding the farm animals in the morn, collect eggs for their breakfast, pony rides twice a week, there's a lovely indoor soft play area, pool, playground, hot tub for adults, ride-on tractors and scooters for kids etc. Also, lovely beaches within 20 mins drive.
Wringworthy looks great & has availability! I will look into it a bit later today. Kids are 22 months & 5 weeks & we are looking for a couple of weeks in July with Aussie grandparents who are flying over to meet our new baby - they'd quite like to go to Europe but I can't face the hassle with 2 little ones!
we stay at wringworthy holiday cottages which is near Looe in Cornwall on the SE Coast so a bit quieter and less traffic to travel to. it has farm animals, swimming pool play barn and is really geared up for young children have a look here it is cheaper than the other two. if not and you want cornwall then try the tourist board site because these places have all been inspected and you wont end up somewhere dirty. how old are your kids?