Strangely, this was the ONLY source I could find.
https://www.youtube.com/watch?v=BNI9w5FvjrY
(just under 100 minutes; it begins with a slideshow; there's one comment, from her great-nephew)

Aka Cecile Eugenie Seed and Cecile Eugenie Booysen.

She lived in Queensburgh, Kwazulu-Natal, South Africa. (Or Centurion.)

From Contemporary Authors:

Cecile Eugenie Seed, best known under the pseudonym Jenny Seed, writes
of her native South Africa, either in stories set in modern times or
in historical periods, and retells traditional African folktales as
well. Her historical novels portray the experiences and emotions of
young characters, both indigenous and immigrant, as they cope with
historical situations and events. Seed once explained that she began
to write such works for children around the time that she discovered
that her own children's history homework was dull. Seed "began to
delve into old books" in the Durban reference library and "the past
became real" to her. She "found that [history] was not boring at all
but tremendously exciting and filled with real and interesting people
just waiting to be put into books."

Seed's 1971 book The Great Thirst, one of her many historical novels
set in the nineteenth century, chronicles the development of a
conflict between the Nama Hottentots and the Hereros in West Africa in
the 1830s, focusing on a young boy's attempt to avenge his father's
murder...

http://www.abebooks.com/servlet/SearchResults?an=jenny+seed&pics=on&sortby=7&x=0&y=0
(17 covers)

https://www.google.com/search?sca_esv=567998441&tbm=bks&tbm=bks&q=inauthor:%22Jenny+Seed%22&sa=X&ved=2ahUKEwibnN2Rp8OBAxU0lokEHaZzCjoQ9Ah6BAgIEAU&biw=1366&bih=644&dpr=1
(some book descriptions)

https://www.kirkusreviews.com/search/books/?q=jenny%20seed&sf=t
(some Kirkus reviews)

https://www.goodreads.com/author/show/225321.Jenny_Seed
(reader reviews)

WRITINGS:

FICTION; UNDER PSEUDONYM JENNY SEED

The Dancing Mule, illustrated by Joan Sirr, Nelson, 1964.

The Always-Late Train, illustrated by Pieter de Weerdt, Nasionale
Boekhandel, 1965.

Small House, Big Garden, illustrated by Lynette Hemmant, Hamish
Hamilton (London), 1965.

Peter the Gardener, illustrated by Mary Russon, Hamish Hamilton,
1966.

Tombi's Song, illustrated by Dugald MacDougall, Hamish Hamilton, 1966,
published as Ntombi's Song, illustrated by Anno Berry, Beacon Press
(Boston), 1989.

To the Rescue, illustrated by Constance Marshall, Hamish Hamilton,
1966.

Stop Those Children!, illustrated by Russon, Hamish Hamilton, 1966.

Timothy and Tinker, illustrated by Hemmant, Hamish Hamilton, 1967.

The River Man, illustrated by MacDougall, Hamish Hamilton, 1968.

The Voice of the Great Elephant, illustrated by Trevor Stubley, Hamish
Hamilton, 1968, Pantheon (New York City), 1969.

Canvas City, illustrated by Hemmant, Hamish Hamilton, 1968.

The Red Dust Soldiers, illustrated by Andrew Sier, Heinemann (London),
1968.

Prince of the Bay, illustrated by Stubley, Hamish Hamilton, 1970,
published as Vengeance of the Zulu King, Pantheon, 1971.

The Great Thirst, illustrated by Stubley, Hamish Hamilton, 1971,
Bradbury (Scarsdale, NY), 1973.

The Broken Spear, illustrated by Stubley, Hamish Hamilton, 1972.

Warriors on the Hills, illustrated by Pat Ludlow, Abelard Schuman (New
York City), 1975.

The Unknown Land, illustrated by Jael Jordan, Heinemann, 1976.

Strangers in the Land, illustrated by Stubley, Hamish Hamilton, 1977.

The Year One, illustrated by Susan Sansome, Hamish Hamilton, 1981.

The Policeman's Button, illustrated by Joy Pritchard, Human & Rousseau
(South Africa), 1981.

Gold Dust, illustrated by Bill le Fever, Hamish Hamilton, 1982.

The New Fire, illustrated by Mario Sickle, Human & Rousseau, 1983.

The 59 Cats, illustrated by Alida Carpenter, Daan Retief, 1983.

The Shell, illustrated by Ann Walton, Daan Retief, 1983.

The Sad Cat, illustrated by Marlize Groenewald, Daan Retief, 1984.

The Karoo Hen, illustrated by A. Venter, Daan Retief, 1984.

The Disappearing Rabbit, illustrated by Walton, Daan Retief, 1984.

Big Boy's Work, illustrated by Paula Collins, Daan Retief, 1984.

The Spy Hill, illustrated by Nelda Vermaak, Human & Rousseau, 1984.

The Lost Prince, illustrated by Walton, Daan Retief, 1985.

Day of the Dragon, illustrated by Collins, Daan Retief, 1985.

Bouncy Lizzie, illustrated by Esther Boshoff, Daan Retief, 1985.

The Strange Blackbird, illustrated by Hettie Saaiman, Daan Retief,
1986.

The Far-Away Valley, illustrated by Joan Rankin, Daan Retief, 1987.

The Christmas Bells, illustrated by Saaiman, Daan Retief, 1987.

Place among the Stones, illustrated by Helmut Starcke, Tafelberg
(South Africa), 1987.

The Station-Master's Hen, illustrated by Elizabeth de Villiers, Human
& Rousseau, 1987.

The Corner Cat, illustrated by de Villiers, Human & Rousseau, 1987.

Hurry, Hurry, Sibusiso, illustrated by Cornelia Holm, Daan Retief,
1988.

The Big Pumpkin, illustrated by Berry, Human & Rousseau, 1989.

Stowaway to Nowhere, Tafelberg, 1990.

Nobody's Cat, illustrated by Alida Bothma, Human & Rousseau, 1990.

The Wind's Song, illustrated by Rankin, Daan Retief, 1991.

The Hungry People, Tafelberg, 1992.

Old Grandfather Mantis, Tafelberg, 1992.

A Time to Scatter Stones, Macmillan (New York City), 1993.

Eyes of a Toad, Macmillan, 1993.

Run, Run, White Hen, Oxford University Press (New York City), 1994.

Lucky Boy, Excellentia Publishers, 1995.

The Strange Large Egg, illustrated by Lyn Gilbert, Gecko Books
(Durban, South Africa), 1996.

Phuthuma, phuthuma, Sibusiso!, illustrated by Cornelia Holm, Kagiso
(Pretoria, South Africa), 1996.

FOLKTALES; UNDER PSEUDONYM JENNY SEED

Kulumi the Brave: A Zulu Tale, illustrated by Stubley, Hamish
Hamilton, World, 1970.

The Sly Green Lizard (Zulu folktale), illustrated by Graham Humphreys,
Hamish Hamilton, 1973.

The Bushman's Dream: African Tales of the Creation, illustrated by
Bernard Brett, Hamish Hamilton, 1974, Bradbury, 1975.