First of all, other Half-Species in the Potter universe are fertile:
“An ’air from ze ’ead of a veela,” said Fleur. “One of my
Harry thought Flint looked as if he had some troll blood in him. (PS)
“Told them I was Stan Shunpike. First person I could think
“And they believed that?”
“They weren’t the brightest. One of them was definitely part troll,
the smell off him. . . .”
Some and Part imply less than half is a possibility.
So we see that according to JKR, when magic humanoids interbreed with wizards, they're fertile. There's no reason to assume giants are different.
But theres also another reason to assume that there fertile, from a biological perspective: Due to survival of the fittest, almost no animal will naturally mate with a specie from which it cannot produce offspring (This is the reason that nearly all hybrids [such as mules and camas] are created by human intervention - otherwise they would not mate.) Since we know Giants will mate with humans without human's forceful intervention, it is safe to assume that their offspring can be fertile.