Another option in Southwest Mass is Race Brook Falls. Here's an old TR:
http://hiking.alpinezone.com/reports/race.htm
More info:
http://www.berkshirehiking.com/hikes...ook.html#trees
Up and back to Mount Race is 6 miles, but the falls are only a bit over a mile up. It's a moderate climb, but if your oldest did a 3 mile hike, she could probably swing it. There's also the Cathedral Pines up by Mohawk:
http://www.berkshirehiking.com/hikes...ral_pines.html