There is absolutely no legal obligation for you to go with the child's mother. In fact, it can be to your detriment, even if all you do is deny the allegations yet again.
Allegations of improper sexual conduct and child molestation is serious and depending on the specific allegations, can be charged in a way so that you face years, if not life in prison. Anything you say at this therapy session would NOT be privileged, as the presence of anyone but a therapist and the patient breaks the confidential nature of the session. Even if you denied any allegation against you, if there was anything that was in any way inconsistent (or even a perception of inconsistency, even if it was a mistake on the part of the listener as to what you truly said), it can be seen as incriminating.
I know the nurturing side of you wants to clear your name and help get past whatever caused this child to make false allegations against you, but trust me - this is beyond just going to a meeting with a therapist.
Going to the meeting and denying it will not convince the mother either. If she's convinced you did something wrong, denials aren't going to change her mind, no matter what the setting.
My best advice to you - politely decline this invitation to the therapy session. Instead, set up your own meeting - with a good criminal defense attorney in your area. Even if there are currently no charges pending, I have a feeling there is or will be an investigation. You need to protect yourself. An attorney can give you more specific advice and assistance.
Best of luck to you.
I hope you took Mr. Dane's advice and consulted an experienced criminal defense attorney. You will almost surely need the help. If you have not, you are free to give me a call, as I am in Marin County as well. The consultation is free with no obligation, and it might help you form a plan of action. The one WRONG plan is to just sit passively and wait for something to happen. You can also read more about false accusations and what to do when you are being investigated here:
Sign up to receive a 3-part series of useful information and advice about child custody law.