I still feel that a traditional family is a mom and a dad and the children and everybody's happy and smiling and together." This is a common sentiment. But, like the divorced mother who expresses it in "Mommy, Daddy and Me" -- a WETA program scheduled for tonight -- many Americans find themselves living in quite different circumstances. The gap between imagination and reality can mean that social policies are poorly designed to help real families get along.

When President Reagan unveiled his tax-revision plan a few weeks ago, he remarked that "there is no cultural institution as enobling as family life," and called his plan the "strongest pro-family initiative in postwar history." But, as he soon learned, the family composed of a working daddy, a stay-at- home mommy and an appropriate complement of children -- the type his plan would favor -- is far from typical. Only 15 percent of all families in this country fit this mold. Even among two-parent families with children, fewer than two of five have only the father working.

The "non-traditional" family arrangements that are now the rule carry certain stresses and strains. Divorce, as WETA's program points out, almost inevitably upsets children. A single mother struggling to care for and support children on her own may work from dawn to midnight and feel very inadequate for her role.

Not all the changes are negative, however. The children in a family where both parents work, such as the Pences shown in the program, may find no one home upon return from school. But they may also enjoy more attention from this "new" sort of father, who feels a direct involvement in his children's upbringing. Even their working mother may be more attentive to their individual needs than the overburdened housekeeper or farm woman of yore.

Divorce may also be better than abuse. "I think divorce is better than living in the house . . . and all the time fighting," one child puts it. Economic necessity or rural isolation forced many spouses and children to endure ghastly arrangements more frequently than many people like to remember. If, like the divorced couple shown in the program, parents understand that their children need to stay close to both of them, the children can even benefit from having two supportive families.

Some things don't change. To a child, as the program's theme suggests, the most important people will always be "mommy, daddy and me." But those people don't have to live in a single arrangement for family life to be "enobling." Policies ought not to be designed with only one type of family in mind. High-Flying Dollars

Everybody knows that the U.S. dollar is gr