The “Tuff Kids Polar Bear Run and Plunge’’ will be held Saturday through the Boston Sports Club at 750 Union St. Preregistration opens at 10 a.m., with the race and accompanying party starting at 11 a.m. The indoor land and water fitness challenge is open to children ages 4 to 13, and nonmembers are welcome. The entry fee is $35; a portion of the proceeds will benefit the JDRF, previously known as the Juvenile Diabetes Research Foundation. To register, visit www.mysportsclubs.com or visit the Sports Clubs for Kids desk at the club. For more information, call 508-528-5960.