Modern grandparenting has taken a new twist which hasn't yet been covered by the how-to-do it books.

ywhen our children were small, a special event occurred every summer. That was when the grandparents volunteered to baby-sit for a few days so we could repair our frazzled nerves, renew our energies and thereby save our sanity.

On the appointed day, we arrived at Grandma's place, car piled high with equipment. We unloaded cribs, a high chair, potty seat, toys, security blankets, the baby dishes and enough food supplies to last for weeks.

The final briefing included names of doctors, a list of telephone numbers for any eventuality and where we could be reached in extreme emergency.

Then came the leave-taking.

There were kisses and waves and we were finally on our way to the great escape.

The scene shifts.

These children are now grown. Circumstances have changed considerably in the light of women's lib and child-bearing being postponed. Somewhere along the way, all of our children have acquired cats. Censusd figures for the '80s might well show that the average household now consists of a male and female person and 2.1 kittens.

Last week my husband and I agreed to baby-sit two of the grandcats.

Our daughter and her husband arrived at night, each bearing a cat carrier; glinting eyes peered out at us through the peep holes of their cases. The nervous animals were let out to explore their new abode while the car was being unloaded. Beds, litter box, toys, individual feeding dishes, grooming brushes and an exotic food supply were carried into the house.

We were instructed on the food preferences of each. In the morning on our bulletin board names and telephone numbers of several vets. A few more instructions and then we were on our own.

The next few days -- and nights -- were very busy indeed. Each morning after their inspection tour, our new boarders dined, then settled down to sleep. They slept all day; prowled all night. Our bedroom was their playland and they purred and pounced on our beds, most often between midnight and 2 or 3 a.m.

During a lull day I consulted my long-time friend on the telephone. She has a wealth of knowledge about these modern grandparenting techniques as she has been sitting her daughter's four cats. My friend is most reassuring about things like hair balls and fleas.

One day the gas man came to our door and when he opened it, Tabby ran out and under his car. We tried to crawl under to reach the rascal only to have him continue this fun game; he scurried under each neighbor's car in turn.

Fortunately, Tabby tired of his game and allowed himself to be captured and returned to the house.

Evenings we were a domestic scene, relaxing in front of the TV, viewing cat chow commericals. The fluffy one chose Grandpa's knee; Calico curled up beside me on the sofa.

Our house guests settled down to a comfortable routine. One chose to take over my husband's favorite Archie Bunker chair; the other preferred my new sofa pillows.

In due time our daughter and her husband arrived home from the beach tanned and rested and happy to see the frolicking felines. They commented on how wonderfully well the little dears looked, thankfully not noticing the dark circles under our eyes. They packed their car, each scooped up one cat to carry; tails twitched good-bye and they were gone without a backward glance.

Once again quiet settled over our household.

In those years past when we departed Grandma's house, she choked back tears. We got letters telling how lonesome she felt and phone calls about the quietness of the house without the patter of little feet on her kitchen linoleum. It was her custom to leave tiny finger prints on the china cabinet glass until our next visit.

Today I wiped paw prints from my kitchen floor without shedding a tear. I vacuumed fluffy yellow hair from rugs, chairs and sofa. And under my bed I found a little plastic ball with a bell in it.

That explains those mysterious jingling sounds I heard when I awoke at night. Ah, tinkling bells and scampering, scurrying feet. I don't seem to miss them at all!

And besides, I need to get rested up. Next week our son and his wife are arriving with Lucky, the granddog. I keep telling myself, how lucky can I be? And if I have any mixed emotions about all of this, James Herriot reminds me in his latest book that The Lord God Made Them All.