First, yes, family dispute resolution - or mediation - is compulsory before you can take the matter to court because you need to obtain a s60i certificate before an initiating application will be accepted by the court.
Second, a parenting plan is not legally binding, but it does hold a great deal of legal weight if you make an initiating application. Basically, it tells the court 'this is what we have previously agreed to be in the best interests of the child'. When your partner goes to family dispute resolution, there may be a chance that an agreement is not reached, but provided a reasonable effort is made by your partner to reach an agreement, he will receive the appropriate documents he needs to pursue court.
Now, a word of wisdom:
Family dispute resolution is not for show.
I cannot stress enough how critically, vitally important it is to make a genuine effort to reach an agreement for care arrangements outside of court. You do not want to go to court. It is not your only avenue and it is not the best way to build a parenting relationship that meets the needs of the children. Court is extraordinarily daunting, costly and will impact your relationship in ways you cannot even begin to fathom, not to mention create a completely insurmountable rupture in the co-parenting relationship between your partner and his ex, which is not in any way what a child would ask for. Additionally, courts do not want to ever see you for parenting matters. I can assure you that you will never, ever get exactly what you are seeking out of a court because they are unpredictable at the best of times and don't care an inch for the squabbling between your partner and his ex.
So, I say to you - do not go into family dispute resolution just to get a s60i certificate. Go into it with a genuine desire to avoid court. As the partner of an applicant father now in the thick of proceedings for parenting orders, I can tell you that in hindsight, we wished we had done more to try and settle things before going to the courts. Heck, we now have to do those things anyway, except it's cost us a lot more time, money and emotional energy than it otherwise would have.
Now, final note, I am a "step-mother" like you, and my partner's ex-wife also did her utmost to stop me from meeting their daughter, including severing contact completely between said daughter and her dad for a period of time.
Unless the mother has evidence proving that you are a danger to the child, the agreement your partner has signed about the child not meeting you has no real legal weight. Section 65DAE of the Family Law Act 1975 basically says that if parenting orders were to be made, the respective parents have no say over the day-to-day decisions each parent makes while the child is in that parent's care - including who the child is in the company of, aka you. He doesn't need to consult with her and he doesn't need to tell her - it's not her business.
Anyway, I hope this has helped and I genuinely do hope your partner finds a way to work it out outside of court. You will likely say, 'There's just no way, she's just too high conflict', but having thought the same thing myself for a long time, and then today hearing from the judge that the conflict between my partner and his ex was petty enough that he wouldn't make new orders because he still had hope they could sort it out outside of court...I can assure you that it is possible to settle outside of court, and for your own wellbeing, you should make every effort to reach that outcome.
