"Look. Mr and Mrs. Johnson, your daughter was molested, and we need to get this scumbag back in prison." In the case study that begins this way (see Michael Braswell, Larry Miller, and Joycelyn Pollock. Case Studies in Criminal Justice Ethics. Long Grove, IL: Waveland Press), was what Ross did right or wrong? What is the morally right thing for you (Ross's partner) to do in this situation?

Personally, I would say that this is wrong from multiple ethical perspectives, but also that there are other problems in the description of the case.
In the description of the case, it really seems as if the police are not being adequately diligent in their investigation. Given that this occurred in daylight, and that Andrews was a registered sex offender, it is probable that a neighbor might have seen something. Also, there is no mention of trying to obtain a warrant to search Andrews' house, something that might also have provided useful evidence, or to look for electronic evidence such as CCTV footage. From this perspective, Ross appears lazy. Perhaps we should assume, however, that the police have exhausted these avenues and found no further evidence.
Morally, there are several problems here. First, an hysterical child is not absolutely reliable evidence. Scientific study results are mixed, but as there are many documented cases of false memories and claims in children, planting false evidence to convict the man on the basis of the report of a single child who will not testify in court is highly unethical. We have no evidence that Andrews actually committed the crime. Just because Andrews has committed other crimes does not prove beyond reasonable doubt that he committed this one.
Next, if Andrews does get a decent lawyer and the conviction is overturned, all the conviction will have done is given the police a reputation for sloppy work and corruption, making future convictions harder to obtain. The lack of additional and obvious investigative steps suggests that Ross is, in fact, lazy and corrupt, taking illegal shortcuts rather than doing the digging needed to produce real evidence. 
Given that you now know of your partner's illegal acts, the most prudent (and moral) course may be to report him to the internal affairs office. There are several reasons for this. The first is prudential. Now that you know about his misconduct, not reporting it puts you in an awkward legal position. Next, now you know that he is willing to take this sort of shortcut, what is the probability that he will do the same again? Are you willing to be an accomplice to any illegal acts he might commit in the future? Also, what if Andrews did not commit the crime and not only did you imprison someone who had not committed a crime but this easy false conviction has allowed the real rapist to go free and continue raping children? As a police officer, your duty is to uphold the law, not to break it.