Eclipse, firstly just to clarify for others, the kids can’t attend the “Chupah” they can be there before and after.
As far as whether they should or not attend. You have mentioned on various occasions how abusive your EX was and still is with your kids, and how he is NOT doing a good job with them. Why give him more fuel to be angry and resentful towards the kids. If they want to go let them go. If you don’t allow them to go, it will only cause tension between them and their father and they will suffer for it in the end. Even if it is YOUR choice that they don’t go, if he is angry at them and punish them for not going he knows that will hurt you, so what’s the point?