I can't recall ever having to actually state a reason for declining an invitation.
Having given it a bit of thought, I realise I'm not actually curious about why you think it's "extreme", more that you term it a "reaction" when it's no such thing. It's a choice I make, because I have the right to make that choice, not a reaction to anything.
"Extreme" isn't the correct term anyway. It's probably "atypical" in that most people have no issues being around children so will happily attend these sorts of events, but it's a binary choice to either go, or not go, and choosing not to go is not "extreme".
It's also not the sole reason why I opt out. As I said, I'm not close to extended family and don't miss them any, so I have no real desire to actually attend in any case and would rather spend a night in the pub with my friends, people I have chosen to spend time with because I like their company, and not just people who happen to be some sort of relative.
I suppose it's a case of not being interested, and inevitably having to put up with children if I chose to go to the bother of faking interest by attending. Again, just no. There are far more enjoyable things I could be doing with my life.