In a quandary over how to amuse the children at their next birthday party? The latest idea for entertaining young partygoers comes from Beverly Kobrinetz, also known as Mrs. K, the "Birthday Baker," who will supervise little hands in baking such goodies as soft pretzels, doughnuts and rye bread turtles and alligators. And she promises that your home won't look like a flour mill afterwards. Kobrinetz gives a two- to three-hour baking lesson to a group of 10 (four years and older) for $50 (460-3191 or 460-0830) . . . Riding a horse sidesadle is a lost art to most people, but not the the hundreds of members of the International Sidesaddle Organization, or to Patty Montali, who coaches women in this vanishing equestrain skill at the Potomac Horse Center in Gaithersburg. Montali says women often learn sidesaddle in order to ride at horse shows or on hunts. Her fee is $15 for a one-hour lesson. Horses and saddles may be rented at the center (301/253-6797) . . . Among the decorating suggestions included in the latest gift catalogue of the National Trust for Historic Preservation are stencil kits, inspired by the early American practice of painting designs through stencils onto walls. Choose from two wall stencil kits ($35 and $40) and two "floorcloth" kits for stenciling a canvas rug ($64 and $83, including canvas). Each has patterns, instructions and a brush (you supply the paint). Order them from the catalogue (50 cents from Preservation Shops, National Trust for Historic Preservation, 1600 H St. NW, Washington, D.C. 20006), or visit the ship on H Street or at Woodlawn Plantation.