Tony, female turtles do NOT return to their eggs, and the males have nothing to do with either mother or babies after the mating. That's the real world truth about them.
The Discworld Companion says it's sex is unknown. However, the description I think that Steven Briggs is wrong about this. In The Light Fantastic A'Tuin is leaving the depths of space and "fighting the tormenting pressures of the solar shadows. In the next paragraph, Great A'Tuin knows what will happen to Discworld if the world is too long in the shallows, but A'tuin can recall doing all this before, many thousands of years ago.
Then at the end of LF, there is a description of A'Tuin looking expectantly at the eight spheres endlessly orbiting the star, on the very beaches of space. And of course the eggs hatch, and the 8 miniature versions of A'tuin, the elephants, and a primitive stage of Discworld come into view. Carefully, A'Tuin turns, and "with considerable relief set out on the long swim to the blessedly cool, bottomless depths of space.
The young turtles followed, orbiting their parent."
The incredible thing about real world sea turtles is that using the earth's magnetic field to steer by, they return to the beach where they were born, to lay the their eggs--and they do this over and over.
I think Terry has copped out by calling A'Tuin simply, "their parent". While obviously, this is a literary work and he can do anything he wants with it, but this description of returning to a place she had been many thousands of years ago on a solar beach is so close to the habits of all female sea turtles, that I think we must really say that A'Tuin is female.