Sit-down snacks are the ace in the hole of beleaguered parents and, in fact, of anyone who eats! Consider this scenario: Your child gets down from the table having eaten little or nothing. He is back 5 minutes later, wanting a cookie. Or this one: He has eaten well, but happens to think, ''cookie,'' and starts begging. Or this one: He seems perfectly comfortable with not eating much, but you worry that he won't make it until the next meal. Consider the school-age child, who comes home famished and eats more or less continuously until dinner. Consider your own scenario: Lunch wasn't too good or too filling. Or this one: You happen to think ''cookie,'' or ''soda,'' or ''latte.'' Or this one: You were so busy at breakfast time that you didn't eat much and now you are starved.

The answer is the same for all of you: the planned snack. Keep in mind, snacks are little meals, not just food handouts. Say to your young child (or to yourself), ''we just ate, but snack time is before long.'' Say to your school-age child, ''sit down and eat your snack right now so you don't spoil your dinner.''

Here is what to keep in mind about snacks:

• Sit to snack, don't allow yourself or your child to eat on the run or eat along with other activities.
• Have snacks be sustaining: Include 2 or 3 foods. Include protein, fat, and carbohydrate.
• Time snacks a long-enough time before the next meal so you or your child have time to get hungry again by mealtime.
• Use snack time to work in foods you didn't get otherwise, such as vegetables or ''forbidden'' food.