You could speak to police and make a complaint of harassment in the present, against her and her partner and the sex-offender ex. None of them care about you and all are bullying you for their own purposes. You're doing nothing wrong regarding your own DC so ignore that. It's just words deliberately calculated to hurt you and scare you into doing what they want.
They're all a POS as far as I can see. Her for not giving a damn about you back then, for having a child with a paedophile and for bullying you now. Her partner for bullying you now. And the ex for being a paedophile and for bullying you now. None of them come out of this looking good. It's their son I feel sorry for, he's the innocent victim in the situation ( just as you were back then). He's not your responsibility though. Legally you owe none of them anything.
Some might call you selfish for not reporting. Maybe they'd be right and maybe they'd be wrong, I can see both sides of the argument. But let's say for arguments sake that anyone saying that is right. Well, being selfish isn't a crime. And it's the same thing the woman and her ex did, when he SA you and she ignored it and had a child with him. They're still being selfish now.
Your responsibility is to yourself and your DC wellbeing, nobody else. Any sense of responsibility you do/don't feel towards society in general, and the boy at the heart of this in particular, is optional. There's no rules for it. There's not only one single morality and anyone outside of it is w.r.o.n.g. Morality is subjective and based on personal values and opinions. Everyone will have their own version of what they're comfortable with, what they consider right and wrong. We live in a free country, which gives you the right to make those decisions for yourself, just like everyone else does.
One thing nobody is taking into account, including the police it seems, is that just because this happened to you a long time ago it doesn't necessarily mean you're over it, have processed it or are fine to talk about it. Maybe the police, at least, need reminding of this.
I'm always amazed at the casual way officials are happy to just come out asking you about any previous SA, when gathering background information. It's not like asking your age or previous address. It's a deeply personal question that ought to be handled with some sensitivity and compassion.
Passage of time doesn't automatically equal, or correlate to, victims becoming "ok"/at peace/somehow "fine" with what happened. Anyone wanting to speak to you about this should be approaching you having at least acknowledged to themselves the possibility that you've never spoken to another living soul about what happened, that you may have successfully pushed it to the back of your mind and forged a life for yourself that works, and that bringing it up to you in even the most sensitive way has the potential to completely derail both your mental health and the life you've built. If someone can't hold those possibilities in their mind when they approach you to speak about it then IMO they've no business approaching you at all.
You could just get a new number. IDK why people don't do that more often. It's not that big a deal to tell people/organisations your new number IMO. It won't get rid of the police though, not if they have your full name or an address from your phone contract records.
It is one of the main benefits of mobile phones IMO. Back in the days of landlines you'd have to either move home or convince the phone company you were being harassed, to get issued with a new number. Now all you have to do is spend 99p on a new SIM card. You can even get dual SIM mobiles now. One number for anyone official, that you never have to change. One number for friends and family, then if you get a problematic person just switch out the SIM card to prevent that problematic person from contacting you again. Just an idea.