Not every event is going to be child-friendly or child-inclusive.
Most people who host child-free weddings understand that there will be some people who won't be able to attend due to childcare issues. It's a price they're willing to pay for the day of their dreams.
Given how many parents allow (not all, of course) their kids to run riot it's not wholly surprising that many people are opting to have child-free weddings and events.
In the end, it's irrelevant if you think it's "selfish." I think it's selfish of you to try and make someone else's wedding all about you and yours and throwing a tantrum because you've been told no-but there you go.
The only choice you have to make is whether or not you're going to attend. If you can't for childcare reasons, don't go. I'm sure they'll survive without your presence for one day.
Since you have a partner, why can't they look after your children and you go to the wedding if you want to go so bad? Or is it that you want to go only on your terms? As in you're a package deal and you and yours go everywhere together?
If so, again, unreasonable levels of selfishness on display from you, OP.
It's a non-negotiable-so come to terms with it and make your decision. You don't get to dictate how other people celebrate their weddings-sadly for you.