Outed at the ticket window

Today I took the two youngest to a local event in our area called "Touch a Truck". The event is put on by United Way and the premise is that kids can climb up and in in all different kinds of trucks. There are fire trucks, buses, construction trucks, tractor trailers and plenty of others. It's pretty fun for the kids and dads LOVE it too!! There's always this moment though when I feel "outed". At the ticket window I'm purchasing tickets everyone over 3 is $5 but when the cashier looks at me, Poo and Mr in the stroller there's a moment of confusion. "Yes, he is 5, yes he is huge and yes he is disabled." This of course isn't what I say - but instead pay the $5 for him. If it wasn't a benefit event for a non-profit I'd probably say something. But i don't. Instead I feel exposed and upset. I shouldn't have to pay anything for him, he's not even going to touch one truck. Actually ill be lucky if we can get out of there without a meltdown from all the loud horn blowing! So why do we go? Because his sister loves it & I want him to be as "normal" as possible. It's just not easy.