Hi Graham,
I am not sure how to put this so apologies if it sounds harsh I do not mean it like that.
At least you were told about her children before you married (I think)

My wife conceived a child by another man after we married and before she eventually came to the UK to join me!

I was pulling my hair out on why the spousal settlement visa was taking so long, it was that she had not put the application in when heavily pregnant, when she did finally arrive here she never said anything about the baby, even when I begged her to come to marriage guidance, when she could have told me.

I would have accepted the baby, it was not his fault, she told lies to cover up lies!

I found out about the baby after I started to divorce her.
Mick.