I think you are BU.
I think you're taking it too personally and you sounds a bit PFB, PSB and PTB tbh!
It sounds like they don't want any children at all - perhaps they feel that's the fairest?
Start making exceptions and it gets complicated and stressful.
Also perhaps they feel if they let your kids stay, then how can they say no to other people's kids?
And then that leads to the danger that guests will drop out of the evening early or the kinds will drift over anyway.
And as you say, you're not particularly close to your brother and you don't sound like you know the bride that well (or even like her tbh)
The MN rule is usually, your wedding, your choice but don't expect people to come - which they don't!
You have the option of going by yourself, and as it's your family, you won't be alone!
Don't make this wedding about you and your kids, let them have it their way with good grace!