Fine to invite the dad but really unkind to make you sit together, particularly under the guise of it being traditional when it isn't.
Glad that your dd has had a go at her brother about this.
But I think you need to lay your cards out on the table to your ds, dil and her parents if they are involved in the table plan too, and ask your ds what his recollection is of why you got divorced - and point out that he was little, you tried to shield him from the worst of his father's abuse and the living hell that he put you through during the marriage and afterwards (including the financial abuse), and that it's really unfair of him to make you sit next to someone who you were hoping you would never have to speak to again, who still gives you nightmares and who you still don't trust.
If the bride's parents aren't involved in the seating plan, then I would be tempted to ring them up and have a chat if you have any sort of relationship with them (even if you just know their telephone number would count!) and see if you can have a word with them. You might find that they want to sit together rather than wanting to sit with you and your ex, so have been pushing this 'traditional' seating plan on their dd as a nicer way to do it for them - which of course it would be as I'm guessing that nobody is looking forward to sitting next to your ex...
however if they know people who have civilised divorces and remain friends afterwards, or your shielding your ds worked really well so he doesn't remember how horrendous your ex was, they may well not be able to imagine how awful a time you had and how horrendous it would be for you to have to sit next to him. And thus, they think it's perfectly reasonable for you to 'pull yourself together and be civilised' for the meal...
I know it will be embarrassing to talk to them about this, but it might be worth a shot. You could also rope them in to help rescue you if they should see your ex heading towards you or talking to you - I know that you will have your dh, dd and others there looking out for you - but it might help to get over to them how bad he is, how bad this will be for you, and how it's not really tenable.
And if they want the meal seating to be traditional like this then you need to think in advance about the church/venue seating in case they want that traditional too - so that you can be on a different pew at least from your ex - you don't want to get there and discover that a seat has been saved on the front row with your name on it, next to one with your ex's name on and your dh/dd/etc sitting several rows away...