Hmm... did you try Ocean Spray in Middleborough? I had no idea any of the growers even offered tours and I've lived on the South Coast nearly my entire life.
a quick google search lead me to this site:
Cape Farm Cranberry Bog Tour, Harwich Cape Cod, Riding Tour which may help. i would also suggest just driving through Middleborough, Freetown, Lakeville, Carver, and Wareham and taking a look. It could be a real pretty afternoon. Depending on how old your children are, the may also enjoy Edaville Railroad (
Edaville USA home ) while you're in the area. Next year, take the train during harvest season, it's absolutely beautiful:
good luck.