Lyme Park at Stockport way? If it is the same national trust property we went too there was a load of dog sh!te in the car park grounds that was available to access by the public (sounds snobby!!!). We went inside the grounds as members of NT and while there was no playground, there was the best lawn I have ever walked on. If it's not too far for you, Nostell Priory at Wakefield has a great playground for the nippers.