Sorry to hear she's been trying to poison the children against you. Make a diary note of what was said. I'm not sure you can do anything at this stage. Best way to handle it is to keep the kids out of the middle and reassure them that this must be a mistake and there is nothing for them to worry about as these are adult matters. What they hear from you is important, don't resort to criticizing the ex back.
Re the solicitors, I would just respond briefly (and your response will also be evidence for later) Eg
"Thank you for your letter. It is untrue that I tried to call Mrs Ex's phone twice on Tuesday - she must be mistaken. And I am not in breach of the undertaking. My phone records will verify this. "
I thought it was an undertaking? Or is it actually a non molestation order? If it is then adjust the wording appropriately, but a short simple, polite denial.
Re the solicitors, I would just respond briefly (and your response will also be evidence for later) Eg
"Thank you for your letter. It is untrue that I tried to call Mrs Ex's phone twice on Tuesday - she must be mistaken. And I am not in breach of the undertaking. My phone records will verify this. "
I thought it was an undertaking? Or is it actually a non molestation order? If it is then adjust the wording appropriately, but a short simple, polite denial.