OP if she's messaged you on Facebook, then maybe you could politely and calmly message back, saying you appreciate she is upset and that that was never your intention, and that you understood that they haven't been together for several years, and can she explain what about it troubles her so much?
Try to be polite and calm and civilised. Say you know she's a great mum as her kids always do her such credit, and that you have never intended to cause her, or them, any issues at all. Ask if maybe you could meet up for a coffee and talk about what could be done to make her more comfortable with it all, and if her concerns could be allayed. That sort of thing.
Apart from anything else, if this does reach a court and your messages are like that, while hers are abusive and seeking to blackmail with the kids as human collateral, then it won't look great for her, will it. And the optimal outcome would be that this has been a horrible shock for her, but she calms down, sees you as a human being who won't mistreat her kids, and you can reach a workable solution.
Please can you clarify this?
I think there was an incident with social services over her stepson about 10 years ago but since then he has paid and seen those 3 kids regularly
Why were social services involved? Over whom? For what? That implies he did something that caused social services to become involved with the family, and if it were 10 years ago and the stepson is still a child, then presumably he was primary - possibly even infant - age?