Filling in as an adopted grandparent

''It's one of the best things I ever did in my life,'' says 81-year-old Ann Pumphrey about her decision to become an adopted grandparent. ''I spent 43 years raising three children by myself after my husband died, and that kind of responsibility teaches you a lot about sharing. When you have children, you just have to share.''

Although Mrs. Pumphrey already has 20 grandchildren of her own, when she was introduced to Mary and George Murphy three years ago, she jumped at the chance to become a substitute grandmother for their five children, age 6 to 14. Because their only living grandparent is in Ireland, the children rarely get to see her - and transatlantic phone calls aren't the same as sharing a game of cards and a plate of cookies.

Nowadays the Murphy children stop by Mrs. Pumphrey's to visit several times a week on their way home from school. They sleep over at her house, share their birthday celebrations with her, go shopping with her, and ask her endless questions about her own growing-up days on a farm in Canada.

''They're fascinated with my childhood, since it was so different from theirs ,'' Mrs. Pumphrey explains. ''I tell them how my grandfather, who had nine sons and two daughters, would start out early each morning to visit all his grandchildren. I always had a cup of tea and piece of cake ready for him, and I looked forward to his visits so much.''

To help fill the vacancies created when today's mobile family members are separated from one another, many community agencies are trying to establish new family relationships by linking up interested young parents and single elderly persons. Grandparent-adoption programs, such as the one sponsored by Family Counseling & Guidance Centers Inc. that brought Mrs. Pumphrey and the Murphys together, are an increasingly popular program.

''When I tell some of my friends about it, they're not too interested,'' Mrs. Pumphrey says. ''They think they might have to do babysitting chores or something like that. I tell them, 'No, it's not like that at all - they just come to visit, and they do more for me than I do for them.' ''

She stops to enjoy a bubbling laugh at her own expense. ''You see, there's a method in my madness!''

According to the Murphys, the giving goes a long distance in both directions.

''Before we met Mrs. Pumphrey, I was on the verge of going to a nursing home in the area to see if there were some elderly people my children could visit,'' Mary Murphy explains in a beguiling Irish brogue. ''I thought it was important for them to have that kind of relationship with someone of another generation.

''Now she has such a special love and affection for the children. It's in how she treats them and how she talks with them. Even though they still don't call her 'grandmother' for some reason, they share an incredible affection. I wouldn't have wanted them to miss out on that for anything.''