Hi Darren,
I have read some of your concerns and can understand how stressful and frustrating this is. You need to stay calm and not make any mistakes that CAFCASS will pounce on and highlight it against you.
Your case is very simple, the kids are already in your custody and you have proven you are a capable father and have your children best interest in mind. Your children are also grown up and at a maturity age, they can make a decision for themselves and you are fully entitled to honour their wishes.
However I don’t know what is going on with you or the mother, and it is unusual for a mother abandon her kids and will need some more detail about the history. The court will never deny access to a parent especially to a mother from seeing their children unless this expose danger to the children in question.
Now by telling the court/CAFCASS that you are encouraging or using your powers to stop the mother from seeing her son is very unreasonable behaviour in their eye. She is the mother, gave birth to the children and she will have some attachment to her children. Yes your ex does have limitations as a mother and needs further help.
Please feel free to call me and we can explore further how to resolve this issue in a civilised manner.
Regards
Nas