Tickets for the theater run $25 dollars a pair. A "moderate" dinner at a local restaurant will be about $30. Add money for parking or cabs. If you have children, throw in a few bucks for a 15-year-old sitter.

Suddenly, your bank account reads "tilt" and you end up spending Saturday night in front of the tube.

There's not much that you can do about the price of tickets, meals or transportation, but more and more families in the Washington area are cutting costs for sitters by organizing baby-sitting cooperatives.

The baby-sitting co-op is the logical extension of the barter system and the first-cousin to the food co-op. No money is exchanged, only children and phony script.

Our co-op began over a year ago and has doubled in size. I read about it in an ad in a local shoppers' guide. Others answered a sign that had been placed on a grocery-store bulletin board: "Families wanted for North Cleveland Park baby-sitting co-op. Boundaries are north of Porter, south of Jennifer and between Wisconsin and Connecticut."

Seven families showed up at the first meeting. We elected officers and set down some basic rules. Our medium of exchange is printed script, each piece equal to half an hour of sitting time. Every family is lent 50 pieces on joining and it must be returned when they leave.

Friday and Saturday nights are time-and-a-half and double-time is charged after 1 a.m. During the day a child is dropped at the sitters; in the evening the sitter comes to the child.

At sitter is reserved through a rotating secretary who calls the list of co-op members and arranges the sit. For this service and for compiling and sending out a monthly newsletter, the secretary earns extra script.

The rules sound very formal -- and they are. Many families hoard their script as if they were playing a giant game of Monopoly. Some squander it all in a few evenings. In general, families budget their bogus money just as they would real money. But to most parents of small children, co-op script represents something more precious than cash: free time.

For our families the co-op arrangement has had many advantages besides saving on sitters. In an age of many childless couples and decreasing number of ready playmates, the co-op has become our "neighborhood." Friendships have developed between parents, as well as between their children.

Raising a child without an extended family can be a lonely and frightening experience for first-time parents. The co-op provides willing ears and ready advice from other adults who have been through chicken pox, roseola, teething and diaper rash and lived to tell about it.

Sitting for other people's older children gives the parent a preview of what's in store. There's no way to stop your angelic 6-month-old from progressing to the Terrible Two's, but if you're prepared, the transition may be easier. A visiting 4-year-old may keep a baby so occupied that even the sitter can get a little peace.

One of my most pleasant afternoons was with 3-year-old Alexa. During my toddler's nap, Alexa and I made oatmeal cookies while she told me all about her imaginary friends. At the end of the afternoon Alexa went home with a package of cookies "for her friends." I was richer from the experience, and by six pieces of script.

No matter how good your 15- or 16-year-old baby sitter is, she's still little more than a child herself. Anyone knows this who has come home to a sink full of dishes and an empty refrigerator from their sitter's "snack" or tried to call home and found the phone tied up for hours.

Co-op sitters are always parents themselves very often fathers sit in the evenings). It is reassuring to go out knowing that you child is with an adult rather than a teen on a meter.

A cooperative, of course, means that you have to sit as well as be sat for. Some parents would rather pay for their free time and not have to reciprocate. But for most, a baby-sitting cooperative provides not only child care, but friends, guidance and reassurance.

Harried parents of young children need that even more than do their kids. Organizing a Co-Op

If you're considering a baby-sitting Co-op:

Formalize your co-op bylaws and make sure every family has a copy.

Every three or four months distribute an updated list of co-op members. Be sure to include the names and ages of children in each family as well as emergency numbers.

Send out a newsletter once a month with everyone's script count. The newsletter also can include a formal welcome to new members and a farewell to old ones.

Set a limit on co-op size. Your children will "sit" easier with a familiar face and so will you.

Get together twice a year, once formally and once informally. The formal meeting is to elect new officers and amend the bylaws. The informal meeting -- including children -- is fun.