Of all the holidays, Thanksgiving is the one in which participation by all in the preparation is most implicit in the celebration. Especially children. Even the youngest can handle a more satisfying role than setting the table.

With four children - ages 3 1/2, 6 1/2, 7 1/2 and 10 - we prepared a traditional menu, using ingredients reminiscent of that first November feast: barley and clam bisque, harvest casserole, corn pudding, two kinds of cranberry sauce, chestnut stuffing and bread stuffing, cranberry muffins and pumpkin parfait.

We found out right away that you've got to start early and allow extra time because the attention span of small fry is no more than an hour and a half and you've got to play musical chores to keep the game going. The soup, stuffings and cranberry sauces are good candidates for advance preparation; they need simmering and jelling time and they keep.

During the hour the barley is simmering in the clam broth, sit around the dining room table or in a circle on the kitchen floor and shell the boiled chestnuts for the next day's stuffing. Even a toddler can shell a few of the lustrous nuts and scrape the meats into the communal bowl. Surreptitous nibbles of the hot chestnuts and a race to pile up the highest tower of shells speed and sweeten the task. Six-year-old Mike pocketed a few unopened chestnuts, unable to resist their soft glowing allure. Next he tore apart bread for stuffing as though it were a stack of math papers.

Like shelling chestnuts and shredding bread, washing and sorting cranberries makes a game. Tiny but determined, three-year-old Miai lavishly bathed the cranberries, so nice to play with, and reluctantly banished the bruised fruit down the garbage disposal.

The oldest child in the group, 10-year-old Melissa, easily handled such chores around the range as melting butter, sauteing the chopped onion for the stuffing and supervising seven-year-old Kate, who stood on a sturdy stool carefully stirring the bubbling cranberry sauce.

Thanksgiving morning's work takes more precision, so, while the roasting turkeys perfumed the kitchen, we laid out areas for each recipe with appropriate teams for each concoction depending on the skills required.

Our 10-year-old pilgrim captained the cranberry muffin operation, handling the knifework in chopping the berries. The younger children delighted in measuring and mixing and gloried in the crimson-speckled batter.

For the harvest casserole, Mimi and Kate, swathed in aprons, knelt on chairs before the cutting board and stood vigil over the steaming squashes until they were cool enough to slice. They peeled the tomatoes, studiously sawed them into rings, greased the casserole and arranged the circles into a colorful design more Impressionist than Puritan. By merely handling each child the appropriate measuring cup, each was able to ladle out cream, wine and butter. After Kate solemnly cracked an egg into the bowl, Mimi mixed up the lot and together they poured the custard over their concentric rings. Each was given a piece of cheese to grate and in unison, almost in benediction, they sprinkled the bits upon their effort.

Making the dessert - ladling on the fluffy pumpkin, spooning out the ice cream, licking fingers and spoon - was doing what comes naturally for all hands.

The satisfactions extend to the consumption, too.

Her father's "What delicious muffins, Melissa!" brought blushes of pride. Mimi and Kate, who either survive on air or are secretly fed by the fairies, actually consumed portions of squash, zuchinni and tomatoes because it was their casserole. These recipes make at least 8 servings: HARVEST CASSEROLE 2 crookneck squash 2 medium zuchinni 3 large firm tomatoes Salt and pepper Basil to taste 1 egg beaten 1/2 cup heavy cream 2 tablespoons butter, melted 1/4 cup dry white wine 1/2 cup grated sharp cheddar cheese

Bring a large pot of water to the boil. Drop in squash and zuchinni and cook no more than 3 or 4 minutes so they remain firm. Remove and let cool. Drop in tomatoes just until skin lifts or puckers remove and peel. Slice squash, zuchinni and tomatoes in thin rings and arrange in overlapping circles in a 10-inch round quiche casserole or deep-dish pie plate. Sprinkle each layer with salt, pepper and basil. Stir together egg, cream, butter and wine and pour over vegetables. Sprinkle grated cheese over top of casserole and bake in 350-degree oven for 20 minutes. PUMPKIN PARFAIT 1 quart vanilla ice cream 1 cup cooked or canned pumpkin 4 eggs, separated 1 cup sugar 1/2 cup milk 1/2 teaspoon salt 1/2 teaspoon allspice 1/4 teaspoon nutmeg 1 teaspoon cinnamon 2 tablespoons butter 1 envelope unflavored gelatin 1/4 cup cold water

Cook pumpkin in double boiler for 10 minutes then add egg yolks, 1/2 cup sugar, milk, salt, allspice, nutmeg, cinnamon and butter. Stir and cook slowly for about 5 minutes. Remove from heat and stir in gelatin dissolved in water. Chill for about 1/2 hour and when mixture begins to thicken fold in egg whites that have been beaten into peaks with 1/2 cup sugar. In parfiat glasses, alternate scoops of ice cream with layers of pumpkin. Refrigerate or freeze until needed. Top with whipped cream if desired.