The nannies who worked in the house on the day of the alleged assault said that they were pressured into supporting the allegations to the point where one resigned. They both said that WA and Dylan were not out of their sight long enough for an assault to take place.
That's not true. I know it's often repeated and thus people assume it must be, but if you look at the actual evidence, it's plain not true.
One of the nannies was WA's employee, and fired by MF. She then filed a deposition - which she later withdrew, without explanation - in which she said that she had not herself been present on the day in question, but she had talked with one of the nannies who had been who told her she'd felt pressured etc etc. That is the lone example of a nanny siding with Allen, and as she was paid by him, no longer with the family at the time, not present on the day, contradicted by those who were, and later withdrew the statement anyway, I can't see her evidence as worth much.
One of the nannies who was present that day wrote a memoir a year or two after the end of the custody case, which describes in detail how toxic WA's behaviour to Dylan was. Apparently Dylan would lock herself in the bathroom for hours to avoid him, and she developed an absolute obsession with never being unclothed around anyone nor seeing anyone else unclothed, which worried people. The nanny is clear that she has no idea if Allen abused Dylan or not, but she confirms that Allen disappeared with her for 20 minutes and that this worried her as there was an unwritten household rule that he never be alone with her because he was so obsessive about her and she was scared. She said in a sworn deposition as well as in her memoir that she didn't tell Farrow this at the time, so when Farrow first said Dylan had disclosed abuse, it was in a context where Farrow believed there had been no unsupervised opportunity for it to occur. She was also asked to put underwear on Dylan that day because she had somehow appeared without any.
She further says Allen was emotionally abusive to everyone, especially Mia, and a bad father. She also criticised Farrow... but not for her mothering skills. She criticised her for exposing her kids to Allen's emotional abuse, and particularly in allowing Allen to adopt Dylan aged 6, despite his being in treatment at the time for having inappropriately intense feelings for and demands upon her.
The other babysitter, Alison Strickland, was not an employee of either side: she was the nanny of a friend of Farrow's, and there that day with her own three charges. She didn't say anything to Allen's nanny, and Allen's nanny never claimed she had. She gave a sworn deposition that she'd caught Allen with his face down in Dylan's lap while she stared at the tv with a vacant expression, and it made her so uneasy she talked to her employer about it that evening. Sophie Berge, the French tutor, testified that she'd told Mia Farrow Dylan had no underwear on that afternoon. Farrow told the nanny, Kriste Groteke (who wrote the memoir) to please put some on the child.
Finally, Allen entered into therapy 2 years before the accusations, explicitly because his behaviour with Dylan was seen as inappropriate, smothering and intense by the family therapist who was brought in to treat Satchel/Ronan. The problems with his behaviour towards Dylan weren't suddenly invented. They'd been flagged up for a long time and were in fact seen as serious enough that Allen accepted treatment in dealing with them.
The full transcript of the judgement over the custody suit is here as has been linked already.
It's uncomfortable reading.