The answer to your question will depend on the state in which the probationer or parolee resides.

PROBATION IS NOT COURT. You would not be getting a new charge and therefore it would not be a matter of admissibility in court. You would be failing to meet the terms of your probation. Nice little trick they play there.

I am going to copy and paste a bit from another post I made but you can get the idea.

I am off probation now myself but can convey my experience to you. First off, you have paid your debt and it is a solid fact that sex offenses have the lowest recidivism rate of all crimes, especially with treatment. 

You need to stay focused on your treatment and or probation, everything you do can be of help to you or hurt you. Often they look at the big picture when assessing these matters. If you fail the polygraph but have clean slate otherwise the chances are very low for you to be sent back to prison in most states. Also should you fail, make sure they know you want to work towards passing it and that it is important to you that you be in compliance. Also talk to a lawyer prior to be assured of the limits they can impose on you and what you should do if you do fail, as George said it depends on the state and also how much of a risk they see you as. 

Only in a few states can you challenge the polygraph outright, what many of them do allow is for the scope of the questions to be limited. For instance, some states don't let them pry into things you were not charged with but, instead only if you are adhering to your probation. 

Having said that, here is my deal: When on probation I failed two polygraphs. One was a sexual history and the administrator made a general accusation, saying that I was hiding a hands on offense of some type. I was never accused or charged of that, and refused to admit to something I did not do and can't even imagine anything in my past to that even could be construed as such. It was never brought up again! I suspect it was a fishing expedition.  From that point on I had to do a compliance check. In that one he showed me a little tiny bump on the readout, smirking and said you know what that means? "It means when I asked you if you have been viewing porn, you were deceptive." Well, I had not and more to the point the genius had also asked me in early questions if I had adhered to the terms of my probation and been honest with my probation officer. I asked him when he failed me on that single issue, if we were done as I was not going to admit to something I didn't do and he said "Yes, you are free to leave unless you want to make this right and tell the truth" I got up and left and called my probation officer and told him I failed, I did not wait, in fact if I farted and thought it could be an issue I called my probation officer throughout the duration of my probation because I wanted him to know that I rightfully respected / feared the situation. I asked him how I could be lying about viewing porn if the terms of my probation were that I not, and that I reported to him weekly and stated on paper I had not, AND I PASSED THOSE QUESTIONS (adhered to probation and been honest with my probation officer). He said it was a good point and they let it go. (This is not a guarantee, just my story)

The fist rule of probation is DON'T BRAKE THE RULES. The second rule is get vested in your future and make sure they know you are trying to get your life on track. When people do reoffend it is often when they are at a low point "crappity smack it" attitude and feel no hope or light at the end of the tunnel. If you go down that road, you will get hounded. If you are doing well and fail a polygraph, depending on the supervising agency they will try to be balanced. The third rule is, don't admit to or even suggest "attempt to appease" if you did not do anything wrong, but firmly and respectfully assert yourself. You can also consult an attorney after as well as before you take the polygraph. 

Best of luck to you and I hope you get your life back on track and stay offense free...