@Twoplustwo123 apologies if I've missed this but, but have you actually sat and spoken to him about this event? As in if he would like you to be there if a babysitter wasn't an issue? Do you genuinely feel that he's not bothered either way if you're there, or if he would prefer to just go by himself? How long has it been since you've actually discussed this particular issue? Could it be time to have another conversation about it? When is the event? If you asked him - if in theory - a babysitter could be found would he he happy that you'd be able to go?
I know you say these are his friends, but there are many couples - in reality not all spouses/partners will necessarily be regulars in this friendship group I'd suspect. I'm sure there's even some like you who wouldn't normally go so will be "new" themselves, especially if it as a big occasion that people would maybe bring their other 'alf too, even if they're not normally there for the regular nights out for this group. So, in reality, you'd likely not be the only new spouse to attend. As you've mentioned it's a big event, and possibly one off or not at all often, why not (because it would be a one off) ask a friend, as you say that they would as a one off. Do you not think, that with the worry about this being a crossroads in your relationship, that the "one off" offer may now not be a really important time to use that, so that you can both go. If you approached a friend and asked, and they were happy to do so - especially if they knew why it was important to you - and you told him you now could go, do you think he'd be happy? You'd tell a lot about the state of things by his response to this, I'd imagine. If it's a crossroads/sliding door type moment in your eyes then it's important enough to use your one off babysitter. If friends have their own children (especially also lack childcare) then offer to have them stay over at yours one night in return, for example.
I think it's worth using your one off of this truly is the crossroads that your fear. If now, when? Because if you're right and it might point to leading completely separate lives, and splitting up, then where's the point in needing that one off favour from a friend when you'd be able to go out anyway - whenever your kiss will be at their dad's. So please, do it. Ask him; "If babysitter not a problem would you like me to come too? I'd really enjoy it and think it would be massively beneficial to (y)our own relationship outside of children...". Just a thought. And then maybe when you've had that first night out with you both and see that a babysitter isn't as bad as you think, then maybe it'll make you more likely to consider a paid-for sitter in future, especially if you feel it would save your marriage. Sometimes these can end up being far less important than the end of a relationship.
Apologies for length, didn't mean to write so much and I hope that the app hasn't messed up the structure of paragraphs etc!