If your wife wanted to claim maintenance for her children she would have to apply to the court, not the CSA.
As the children are being maintained by their father, it is unlikely that the claim would be successful, but not impossible. I would say that in view of the children's ages and the length of the marriage you would not be liable for maintenance.
The court would consider that the natural parents are perfectly capable of supporting the children - unless there is a reason (disability?) why the mother cannot go to work.
However, if you are fabulously wealthy and the children have enjoyed a much higher standard of living than their father would have provided, then there is a remote chance of the court awarding maintenance.
Sorry it isn'st a straightforward yes or no answer.