I think it's weird that Grindelwald had no children at all, it would seem suiting that he would have someone who continued his legacy. Therefore I kind of had an idea that he had three children who were all born before or during his rise.

So far I got;

Oldest- Melda Grindelwald(later Travers)

Helped her father to terrorize Europe. After his defeat, she avoided execution because of Dumbledore. He decided that the children of his former friend(lover/crush?) deserved a second chance. She later married into the Travers family and had two children(Gael and Valente). For those who don't know, that family was later associated with Death Eaters. Therefore, she crossed path with Voldemort who wanted her influence to recruit more people to his cause. It did not end well.

Middle- Cade Grindelwald

Was unfortunate enough to be born a squib or a late- bloomer (which is extremely rare). Therefore, he was regarded as little more than worthless by his father. However, being born the first son he always had a strong admiration for Grindelwald. Just as his sister he was taken in by Dumbledore. Living as a squib and son of Grindelwald he wouldn't want to blend in with muggle society but at the same time he is shunned and ridiculed by the purebloods in wizarding society, because of his lack of magic and because of his last name.

Youngest- Alaric Grindelwald

Okay, this one is difficult, he is the youngest and had the most time to adapt to the regular society. He is able to learn that Grindelwald's values might not have been the best. He was offered to study at Hogwarts but declined it because of people speaking of his father. Unlike his older sister, Alaric decided to avoid bad influence, in this case, pureblood fanatics because he didn't want to be associated with his father. It is possible that his relationship with his sister suffered because of this. With the help of Dumbledore, he got a job at Ollivander as his assistant and tried to help his brother(he does not believe that his brother is a squib). He married and had a son (Gage) before Voldemorts rise.