After looking at a number of statistics that don't tackle your question directly, my impression is that being in a single-parent household might be correlated with an important factor of childhood sexual abuse, but that it is not a contributor on its own. Namely, a strong correlate of childhood sexual abuse is involvement of the biological father in raising the child. In families where the parents have a bad relationship (whether they are divorced or not), the most common perpetrators of sexual abuse - male relatives and and acquaintances - have a greater chance of committing sexual abuse. Very often the perpetrator is the mother's partner (whether he is living with them or not), who is not the biological father of the child.
(A few sources on the most common perpetrators: first bullet on the page, bottom of page 8, Finkelhor study mentioned in last bullet.)
Some relevant stastics:
A pediatrics source on general abuse and neglect, that shows that a child living with either a single mother or a mother and her live-in partner, is at greater risk of abuse and neglect:
living with their married biological parents places kids at the lowest
risk for child abuse and neglect, while living with a single parent
and a live-in partner increased the risk of abuse and neglect to more
than eight times that of other children
A source on fatherless kids (which summarizes information from another source which I haven't read myself), that again shows that many victims of childhood sexual abuse are living with the mother's boyfriend or a stepfather:
Sexual abuse. A study of 156 victims of child sexual abuse found that
the majority of the children came from disrupted or single-parent
homes; only 31 percent of the children lived with both biological
parents. Although stepfamilies make up only about 10 percent of all
families, 27 percent of the abused children lived with either a
stepfather or the mother's boyfriend. Source: Beverly Gomes-Schwartz,
Jonathan Horowitz, and Albert P. Cardarelli, "Child Sexual Abuse
Victims and Their Treatment," U.S. Department of Justice, Office of
Juvenile Justice and Delinquency Prevention.
A source that describes how pedophiles work, showing that they need to have ample time alone with the child:
Many times pedophiles will develop a close relationship with a single
parent in order to get close to their children. Once inside the home,
they have many opportunities to manipulate the children -- using
guilt, fear, and love to confuse the child. If the child's parent
works, it offers the pedophile the private time needed to abuse the
child.
So this source does mention single-parent families as likely targets, but if the pedophile becomes the live-in boyfriend in order to be closer to the child, I'm guessing it wouldn't count as a single-parent household anymore?
To answer whether children from single-parent households are more likely victims than children living with both parents, the numbers would need to be broken down differently from what they are in the sources I've found. It seems to me that this information isn't widely available because single-parenting on its own is not an important factor. But obviously, more data is needed to answer that question.
