Congratulations!!!! I hope the talk with the kids goes well.
When I was getting married I posted a similar thread and the commenters here gave me lots of good advice:
https://www.outofthefog.net/forum/index.php?topic=77583.msg675506#msg675506There are also some other older threads about this (and similar things, like how a PD ex might react to you having a baby, I think there are a lot of similarities to that situation as well).
Here is how we did it: We told the kids in the middle of a long stretch with us, while their mom was out of town. The idea was that we could kind of set the tone and they'd have some time to let the good feelings set in before their mom tried to disrupt it. DSD8 lit up, DSS11 had some questions. H emailed BM a one-line note to tell her that we were getting married. I'm not sure he owed it to her but ultimately we felt like if she heard it from the kids she's have a really bad reaction right in front of them. This way she could process her feelings a little bit before she had to address it with the kids.
We were all braced for a temper tantrum and then it was ... nothing. Eerily silent. Nothing from the kids that made it seem like she was going to try to disrupt their happiness. I kept thinking she was plotting something. She had just gotten a new boyfriend so I think that helped distract her.
There was one small incident in the months leading up to the event: She emailed H to say that SS had said he was sad that his parents weren't getting back together. H asked SS who said he had said that. But it never really came up again - at first I thought she was lying but it seems like maybe it was just part of the normal processing for DSS. Either way, both kids seemed overall very happy that we were getting married and they've said several times since that they're glad it happened.
H didn't tell BM anything else - we didn't think she needed the details. But about a month beforehand it seemed like the kids had told her about when it was going to be because she asked for an update. H told her when it would be and that the kids would be flower girl and ring bearer. She really went off the rails in a very unstable way (some of the details in the other post). Again I thought that she probably was going to do something huge like refuse to let the kids go with us or show up at the venue (although we had not told her where it was).
We spent a lot of energy brainstorming all the ways she could try to ruin it. It was kind of helpful because it led us to do stuff like, making sure we had a plan in place for if BM wouldn't turn the kids over to H before the wedding. But ultimately I think I spent too much energy stressing about it and I wish I'd focused more on DH and the planning of the event itself and of the kids. She didn't pull any final weird moves, thankfully.
One thing I'm really glad we did was that we explicitly told the kids that it's ok to have conflicting feelings about it and we told them they could participate in any way they chose. They both ended up wanting to walk down the aisle and stand on the stage with us. But for awhile DSS was saying he didn't want to stand on the stage. Even though it would've been a little weird to have SD up there and SS not up there, we said that was fine. (It turned out he thought he was going to feel awkward about the kiss but we told him that he could make a silly face during it and that cheered him up). We also kept the wedding really lowkey - that's our natures anyway but also it created less pressure for the kids, I think.
Again, congratulations!!! I hope BM doesn't find a way to ruin this time for you - you should be getting to celebrate!