Does Ruffus Humphrey and Lily Van Der Woodsen had a child?

No.

EDIT: *spoiler* Lily and Rufus had a child which Lily secretly adopted to another family. Rufus didn't know about this until Lily's mum told Rufus. Rufus decided he wanted to meet their son so he arranged a meeting with Lily and the son. The man who adopted the boy told them he had been killed in a boating accident. This however was not true but they told them this so they would keep away. Eventually the son starts writing letters and becomes friends with Dan *spoiler*