To not want to attend surprise celebration

[Surprise76596]

PiL live abroad. One has a big birthday coming up. BiL (who lives in same country as them) invited us to join a celebratory weekend. I said (via DH) that I was not comfortable w surprise element. Not a good idea for older people, definitely not right for PiL, and also I think anticipation is part of the pleasure. I was assured that 1 PiL was ‘in on it’ and happy w the plan so went along with it.
Now it turns out that neither PiL knows we are travelling to join them for the w/e (w young children).
I think they won’t like the surprise or the noise of the kids, and I find the idea of ‘popping out and saying hi’ really awkward.
I really like PiL, and BiL so I dont want to be difficult or a party pooper, but nor do I want to spend my weekend travelling for an awkward experience.
AIBU to not want to go?
If not, how do I get out of it diplomatically?

YABU if you think you know how they'll respond. If their two sons think it's OK, then go! Seems to me, you're projecting your anxiety onto them.

Not a good idea for older people,

How old are they? I mean if they're in their 90s, maybe, but really? Ageist, much?

Is there a compromise, to plan the trip and give them a week's notice?

Be honest. This is about you and your anxiety, and you’re pretending it’s concern for your PiL. You don’t want to go so you’re making up all kinds of complicated issues as to why it’s all a bad idea. Just put your big girl pants on and go.