When Mary Poppins takes over the kitchen for a week in Mrs. Brill's absence, she serves just the kind of sensible English fare you'd expect. And her philosophy concerning junior chefs parallels that of this volume's ""culinary consultant,"" Maurice Moore-Betty, who learned that youthful ""enthusiasm had to be regulated at any price."" Certainly any youngster who set out to make Shepherd's Pie from the recipe here would need a firm guiding hand. Even an adult might despair at being told to ""reduce the amount of sugar"" after that oversweetened salad dressing is already mixed. Unfortunately some of the most appealingly typical Poppins specialties won't be too easy to reproduce in American kitchens--gooseberries and quinces aren't on every supermarket shelf and few child cooks will get within striking distance of a six pound rib roast. And though the junket and bread-and-butter pudding should trigger nostalgic reactions (as will, in their own way, the quarts of water and tablespoons of salt used to boil fresh veggies), the collection isn't substantial enough to live up to Moore-Betty's promise of ""gems of Edwardian cookery."" Even seven days' worth of dinner time entertainments--with guest appearances by Admiral Boom, the Topsy-Turvys and others--can't really assuage our disappointment in learning that Mary Poppins uses that newfangled Jello. Which only proves that a good nanny is wasted in the kitchen.