Hi,
I think it is a bit more complicated than many are making out.
He has clearly been bitten once and is now commitment shy.
I see nothing wrong with a prenup, but it should definitely be protecting his existing assets and not the (future) marital assets, especially if you intend to give up working and look after his children.
To reassure you a little, you cannot 'prenup' your (future) childrens' needs and he would have to pay you enough to maintain (roughly) the lifestyle that they enjoyed prior to the divorce, assuming he could afford it.
As the law currently stands, without a prenup, he would have to give you spousal maintenance for the rest of your life and maintain your 'needs', which divorce lawyers have a habit of greatly exaggerating. I can totally see why he would not want to do that, especially if you would not be sacrificing a high flying career to look after the children.
Where I can see where he is coming from is that there is no reason you should become very wealthy merely through spending 10 years of your life with him and having his children.
Fair, to me, is putting you back in the position you would have been having not married him with, of course, excellent provision for both of your children. That would include compensating you for loss of earnings, with reasonable promotions etc and loss of pension contributions. In addition, he would have to deal with your future genuinely reasonable needs.
All the above could be worded in a prenup and a court would take account of it (although it is not absolutely binding in law in the uk).
If he is coming from the above position, you need to have a meaningful conversation with him, having already taken legal advice of your own. If he just thinks he can marry you and have virtually no meaningful commitment to you, then why marry at all?