Hopkinton State Park is a great place to take your children. There is swimming, boating, biking and hiking.  There are 2 beaches where lifeguards are present at all times.  The water is very clean and so are the facilities.  There are lots of trails to hike or bike through the woods or you can just walk on the auto road which is a beautiful walk as well.  Through the summer months, they charge $5/day to get in, or you can buy a season pass for $35.  There is no charge through the winter time.  It is an absolute beautiful, quiet and safe place to walk with children and animals too.  Definitely a place to check out.