One out of six people in the United States is a grandparent, and never have grandparents been more necessary in the family structure. In addition to traditional roles as senior advisers to the family and helpers with parental duties, grandparents today are asked, on occasion, to take over the parent role. This comes about because of single-parent families unable to cope with the stress of child-rearing, illness and death.

Most of the time, grandparents provide a kind of family glue that keeps relatives in touch with one another, and grandparents help with setting the value system for all the family. Dr. T. Berry Brazelton encourages parents and grandparents to work together to bring a harmony to a family and says of grandparents, "Their presence implies an important past, and their beliefs can become a part of the family's belief systems. . . . Strong values are at risk for too many families. Who else but grandparents can keep them alive?"But what few people discuss is what happens when only grandparents are around to keep not just values, but children, alive. What are the problems when the grandparent functions as parent? Here are two letters - one from a grandmother, another from a granddaughter - that highlight the frustrations of raising a child's child.

Dear Lois: Four years ago I married a wonderful man. Shortly after we married we had to take custody of his then-6-year-old grandson. The first year we had him, he was really good. Then once he felt comfortable with us, he became every parent's worst nightmare.

We have had him in counseling for three years. He has come a long way, is now mainstreamed and is getting B's and C's. We received many compliments on his behavior in school and church and at other people's homes, but at home he is a terror. His temper is unreal; he lies, steals and throws temper tantrums like a 2-year-old and is now getting more physically abusive. I'm scared, but my husband and the boy's counselor and school teachers just don't believe it. What can I do?

My daughter watches him while I work, so she has seen him at his worst. She told the counselor, but he didn't believe us. Help! Please.

- Karen from Ohio

Dear Karen: This boy is not the only one who needs help. You do as well, and you need a counselor who will work in concert with the boy's counselor so that together you are able to get at the root of his problem. You didn't give the reasons for taking custody, but he has to have some sense of parental rejection, and only professionals can help.

In terms of dealing with tantrums, look for what psychologists call "storm signals." Before a tantrum there are certain signs, and one must learn to recognize them and thwart the tantrum in the making. Sometimes tantrums are preceded by a child feeling hot. Next comes kicking or hitting or whining. By this time all you can do is help the child get through the storm.

But it certainly sounds as if this child resents your replacing his mother. Since no one other than your daughter is aware of your situation, get some professional help - and tell that "wonderful man" to listen carefully when you tell him your problems. Good luck.

Dear Lois: I am 16, a high school senior and live with my grandparents. I work weekends in the ticket booth of a local dinner theater, have had some bit parts in performances and would love to be an actress.

Three weeks ago, just after intermission, I was robbed while alone in the booth counting receipts. I cooperated but was left tied up and gagged on the floor. I couldn't get free, so it was about 45 minutes before I was found. Now my grandparents won't allow me to go back to work at the theater. I really want to work there. I know my grandparents love me and are worried, but robberies can happen anywhere, and I'm not scared. Being bound and gagged was actually a unique experience for a would-be actress. How can I handle this situation without being confrontational?

- K.L., Fairless Hills, Pa.

Dear K.L.: Honey, I think being bound and gagged and left alone for 45 minutes may be good for an actress, but it's tough on grandparents who are responsible for your well-being. In order to work there again, I suggest you take your grandparents to the theater, have them talk to the manager and have him assure them (and you) that if you return you will a) never be left alone to count receipts or b) all receipts will be taken at 15-minute intervals to the office by someone (preferably someone who's also on the wrestling team).

If the manager can do none of the above, remember that not every actress started life in the ticket booth of a dinner theater.

Dear Lois: Recently my aunt gave two lottery tickets to a friend of hers on her 80th birthday. As you probably guessed, one of them was a winner - $15,000!!! Do you think this woman should have given some amount of the winnings to my aunt?

- John J. McCarthy,

Baldwinsville, N.Y.

Dear John: Doesn't much matter what I think. What does your aunt think? Is she the one who's disappointed that her friend didn't share the winnings - or does she take the view that a gift is a gift, and if she didn't want her friend to win, why did she give her the tickets?

However, I do think it would have been gracious for the friend to buy a gift for your aunt with some of the proceeds.