Is the father doing something sexually with the daughter? If yes, you can report him to the police.
If he is not doing anything sexually with the daughter, there is no law that prohibits a parent from sleeping in the same bed with his child of either gender.
If you are concerned, besides the police, you can report your suspicion the DSHS. DSHS's website is: http://www1.dshs.wa.gov/ca/general/index.asp .
You should think carefully before making a report. You should not be just making a baseless report to get at your husband. An investigation by either the police or DSHS can make everyone's lives difficult, even if the final conclusion is that there is no abuse.
The first question you need to ask yourself is "Why does this bother you?" If it is because you don't think it is appropriate due to some societal expectation, then you want to think twice about reporting it. This may be more about you personally than about the situation.
On the other hand, if your ex-husband has a history, has made comments, or there is something that your daughter has said or done that makes you suspicious as to what is going on, then you want to report what you know to either the police or to child protective services. Be prepared to not only let them know that it is happening, but also to provide any evidence or information you have as to why it is inappropriate.
In the least, you can contact your state's hotline phone number to ask the question and provide whatever additional information you have. They will be able to let you know if there is a violation of any statute or ordinance.