Pretty soon we'll go to a family vacation. There will be 6 families, ranging from 2 to 6 people. 20 people in total.
We will do some activities and a couple of families insist that kids shouldn't pay and the total expenses should be divided between the adults.

This way, a couple with 4 kids will pay the same as a couple with 1 kid, or a single mom with 2 kids will pay half of what a couple with 1 kid will pay (both families have 3 people).....and so on. It's pretty obvious and definitely not fair.

I'm the only one that is opposing and arguing/fighting just few days before we leave. The other families either don't want to get into an argument or don't understand the problem/math.

I tried to explain that they are confusing kids not paying, because their parents pay for them, with kids being free, which is not the case.
I tried to present the math, I tried to give them other examples. Nothing worked. I feel like I'm in an episode of the twilight zone.
The 2 families that insist will benefit the most from this, but I don't want to think that they are dishonest. I really think that they don't understand.

Am I the asshole?
Also, how do I make them understand?