He had two children with Susan Hastings. Though It was not stated on his wiki entry. It was said that they were married and that it ended in divorce. The novel guide makes reference to his divorce. I would have to conclude that he was married and then divorced from the articles I have read.

"As Edwin McDowell reported in the New York Times Book Review (8 Nov 1981), Silverstein "for several years now... has refused interviews and publicity tours, and he even asked his publisher not to give out any biographical information about him." What is known about Silverstein, however, is that he was born in Chicago (Illinois) in 1932, is divorced and has one daughter."

So I think I can say he was married.

His Wikipedia page reports that, though he has two children, he was never married:

Silverstein had two children. His first child was daughter Shoshanna (Shanna), born June 30, 1970, with Susan Hastings. Susan Hastings died 5 years later, on June 29, 1975, in Baltimore, Maryland. Shoshanna's aunt and uncle, Meg and Curtis Marshall, raised Shanna from the age of 5 until her death of a cerebral aneurysm in Baltimore on April 24, 1982, at the age of 11. She was attending the Bryn Mawr School in Baltimore at the time of her death. Silverstein dedicated his 1983 reprint of Who Wants a Cheap Rhinoceros to the Marshalls. A Light in the Attic was dedicated to Shanna, and Silverstein drew the sign with a flower attached. Shoshanna means lily or rose in Hebrew.

Silverstein's other child was his son Matthew, born on November 10, 1983. Silverstein's 1996 Falling Up was dedicated to Matt. Matthew's mother is alleged to be the Sarah mentioned in the other thanks that appears on the dedication page.